The role of the input frequency in L1 Spanish phonological acquisition. A corpus-based study

1. INTRODUCTION

First language (L1 henceforth) acquisition and development have drawn the attention of researchers for centuries. However, new technology development from the last few decades has entailed a qualitative change in research (Dolgova & Tyler, 2019Dolgova, N., & Tyler, A. (2019). Applications of Usage-Based Approaches to Language Teaching. In X. Gao (Ed.), Second Handbook of English Language Teaching (pp. 939-961). Springer. https://doi.org/10.1007/978-3-030-02899-2_49
; Ellis, 2017Ellis, N. C. (2017). Cognition, Corpora, and Computing: Triangulating Research in Usage-Based Language Learning. Language Learning, 67(S1), 40-65. https://doi.org/10.1111/lang.12215
; Kern et al., 2014Kern, S., Gayraud, F., & Chenu, F. (2014). The role of input in early first language morphosyntactic development. Language, Interaction and Acquisition, 5(1), 1-18. https://doi.org/10.1075/lia.5.1.00int
; MacWhinney, 1996MacWhinney, B. (1996). Computational analysis of interactions. In P. Fletcher, & B. MacWhinney (Eds.), The Handbook of child language (pp. 152-178). Blackwell. https://doi.org/10.1111/b.9780631203124.1996.00006.x
). The gradual introduction of new technological tools and the adoption of common methodologies and procedures made the design of the first corpora of child language possible. In those corpora, hundreds of speech recordings from different-aged children were transcribed, providing researchers with an invaluable database for the study of child language (Ellis, 2017Ellis, N. C. (2017). Cognition, Corpora, and Computing: Triangulating Research in Usage-Based Language Learning. Language Learning, 67(S1), 40-65. https://doi.org/10.1111/lang.12215
). Currently, the international corpus of reference is CHILDES¹ https://childes.talkbank.org/ (MacWhinney & Snow, 1985MacWhinney, B., & Snow, C. (1985). The child language data exchange system. Journal of Child Language, 12(2), 271-295. https://doi.org/10.1017/S0305000900006449), a multilingual child language corpus, in which we can find samples of Spanish language, some of which were used to corroborate results from this study. And particularly regarding the phonological treatment of corpora, the development of the software PHON² https://www.phon.ca/phon-manual/index.html (Hedlund & Rose, 2020Hedlund, G., & Rose, Y. (2020). Phon 3.1 [Computer Software]. https://phon.ca.
) meant a landmark in the study of child language.

Within the field of L1 acquisition research, the description of the phonological development involves four basic concerns (Grunwell, 1981Grunwell, P. (1981). The development of phonology: A descriptive profile. First language, 3, 161-191. https://doi.org/10.1177/014272378100200601
): the great variation from one individual to another; the extension and gradual regularisation of the child’s pronunciation system, characterised by unsystematicity; the difficulty determining the starting point of the phonological development; and the need to consider both the input and output in the process of description. Grunwell (1981, p. 167)Grunwell, P. (1981). The development of phonology: A descriptive profile. First language, 3, 161-191. https://doi.org/10.1177/014272378100200601
disapproved of the fact that “studies are to discover when children achieve the correct pronunciation of the sounds of their language”. She considered that the question about when the sounds of speech are learnt was ill posed, due to factors such as the wide range of individual variation, or the fact that a child does not acquire each phoneme separately. Therefore, research on phonological acquisition must not focus so much on the precise moment at which a child acquires a certain phoneme, but on the search for patterns by describing large samples of speech language. “We need models of usage and its effects upon acquisition” (Ellis, 2017, p. 48Ellis, N. C. (2017). Cognition, Corpora, and Computing: Triangulating Research in Usage-Based Language Learning. Language Learning, 67(S1), 40-65. https://doi.org/10.1111/lang.12215
).

This subject matter of phonological acquisition has been largely aimed at improving research on language disorders. From a detailed study of a child’s normal linguistic development and the establishment of patterns in language behaviour it is possible to detect atypical phenomena in the development of an individual. According to Ingram (1976)Ingram, D. (1976). Phonological disability in children. Edwards Arnold.
, the knowledge about patterns of typical language development gives us the clues for the treatment of pathologies. And corpus linguistics plays a pertinent role in this regard, since corpora are a huge source for the analysis of natural language in the elaboration of, for instance, what Acosta and Ramos (1998)Acosta, V., & Ramos, V. (1998). Estudio de los desórdenes del habla infantil desde la perspectiva de los procesos fonológicos. Revista de Logopedia, Fonatría y Audiología, 18, 124-142. https://doi.org/10.1016/S0214-4603(98)75683-9
demanded: a phonological inventory; or to study the role of input in the acquisition process examining child-directed speech (CDS) in natural contexts.

The present study is based on CHIEDE (Garrote, 2010Garrote, M. (2010). Los corpus de habla infantil. Metodología y análisis. Servicio de publicaciones de la Universidad Autónoma de Madrid.
), a cross-sectional corpus in which n=59 children aged 3;0-6;0 participated. The corpus was recorded, transcribed and, subsequently, tagged by means of automatic processing techniques (phonological and morphosyntactic tagging software), and then manually checked to correct possible tagging errors. This methodology facilitates the retrieval of linguistically annotated data (parts of speech, morphological, and phonological information) to quantify linguistic features. It is descriptive work, following an observational method based on performance, on external empirical data, and not on competence and experimentation.

This paper presents a phonological study of L1 Spanish children with the aim to show the phonological development displayed by the participants. Taking into account the participants’ age, our purpose was not to establish the order of acquisition of phonemes, but to carry out a description of the typical phonological development of Spanish-speaking children from 3;0 to 6;0 years old, based on the frequency of occurrence of phonemes (providing a phonological inventory), and to highlight the role of the input frequency as a facilitator to acquire phonemes (even those traditionally considered more complex). Three questions are considered: (1) Is the phonological system completely acquired at 3;0? (2) Is 4;0 a turning point in the acquisition process as many linguistic studies claim (Bosch, 1983Bosch, L. (1983). El desarrollo fonológico infantil: una prueba para su evaluación. Anuario de Psicología, 28(1), 87-114.
; Díez-Itza & Martínez López, 2004Díez-Itza, E., & Martínez López, V. (2004). Las etapas tardías de la adquisición fonológica: procesos de reducción de grupos consonánticos. Anuario de Psicología, 35, 177-202.
; Maratsos, 1974Maratsos, M. P. (1974). Children who get worse at understanding the passive: A replication of Bever. Journal of Psycholinguistic Research, 3(1), 65-74. https://doi.org/10.1007/BF01067222
)? And finally, and most importantly, (3) To what extent is the input frequency relevant in this process? The goal is to clarify these questions through the revision of some of the most significant theories and research, and the analysis of data from different corpora.

2. PREVIOUS RESEARCH

Morphology and syntax are the linguistic levels which have been addressed to the most extent by research on L1 acquisition. Studies carried out on child language have mainly focused on the acquisition of the lexical and grammatical structure, to the detriment of phonology, semantics, or pragmatics. According to Vihman et al. (2009, p. 164)Vihman, M. M., DePaolis, R. A, & Keren-Portnoy, T. (2009). A dynamic systems approach to babbling and words. In E. L. Bavin (Ed.), The Cambridge Handbook of Child Language (pp. 163-182). Cambridge University Press. https://doi.org/10.1017/CBO9780511576164.010
, “The role of phonology in the development of linguistic knowledge is often given short shrift by researchers interested in word learning”. Consequently, phonological studies on acquisition are less frequent (Polo, 2016Polo, N. (2016). La investigación actual sobre el desarrollo de la fonología del español como lengua materna. Lenguas modernas, 47, 137-152.
). Moreover, a vast majority focus on the English language. Though research has been gradually carried out on other languages, it is “heavily biased toward Indo-European languages of Western Europe with the bulk of research still concentrated on English” (Stoll, 2009, p. 89Stoll, S. (2009). Crosslinguistic approaches to language acquisition. In E. L. Bavin (Ed.), The Cambridge Handbook of Child Language (pp. 89-104). Cambridge University Press. https://doi.org/10.1017/CBO9780511576164.006
).

One of the pioneering works on phonological development was Stampe’s (1969)Stampe, D. (1969). The acquisition of phonetic representation. In R. I. Binnick, A. Davidson, G. Green, & J. L. Morgan (Eds.), Papers from the Fifth Regional Meeting of the Chicago Linguistic Society (pp. 443-454). Chicago Linguistic Society.
, for whom the language acquisition process is based upon an innate mechanism children have in order to simplify adult words. By means of these mechanisms or processes -unstressed syllable deletion, clusters reduction, merging vowels into /a/- the child goes from what Stampe called a “language-innocent state” to the adult production.

Later, Ingram (1976)Ingram, D. (1976). Phonological disability in children. Edwards Arnold.
adopted Stampe’s theory for clinical phonology research. Following the piagetian stages (Piaget, 1926Piaget, J. (1926). The language and thought of the child. Kegan Paul, Trench & Trubner.
) of cognitive development and their corresponding linguistic periods, Ingram established a parallelism with the phonological level, thus locating the evolution of the different phonemes and phonological skills at distinct stages from the sensorimotor stage (0;0-1;6) to the formal operational stage (12;0-16;0).

However, crosslinguistic studies on acquisition beyond the early period (around one year of age) have proved that it is not possible to establish clear stages of development applicable to every language. For instance, Durgunoğlu and Öney (1999, p. 283)Durgunoğlu, A. Y., & Öney, B. (1999). A cross-linguistic comparison of phonological awareness and word recognition. Reading and Writing: An Interdisciplinary Journal, 11, 281-299. https://doi.org/10.1023/A:1008093232622
examined the “effects of language-specific influences on the development of phonological awareness” and explained how structural phonetic differences among languages mean differences in the child’s development of phonology. In a similar line, Bleses, Basbøll, Lum and Vach (2010)Bleses, D., Basbøll, H., Lum, J., & Vach, W. (2010). Phonology and lexicon in a cross-linguistic perspective: the importance of phonetics - a commentary on Stoel-Gammon’s “Relationships between lexical and phonological development in young children”. Journal of Child Language ,38(01), 61-68. https://doi.org/10.1017/S0305000910000437
set up a ranking of 7 languages based on the complexity of their phonetic systems (vowel/consonant ratio) and concluded that the most complex one was the Danish phonemic system, followed by the Swedish, the Dutch, the French, the English (American), the Galician and the Croatian. Bernhardt and Stemberger (2017)Bernhardt, B. M., & Stemberger, J. P. (2017). Investigating typical and protracted phonological development across languages. In E. Babatsouli, D. Ingram, & N. Müller (Eds.), Crosslinguistic Encounters in Language Acquisition: Typical and Atypical Development (pp. 71-108). Multilingual Matters. https://doi.org/10.21832/9781783099092-008
, comparing typical development with protracted phonological development, showed that in four languages, Mandarin, Arabic, Slovene and European Spanish, the WWM³WWM stands for “whole word match”, that is, the child’s pronunciation equals the adult’s. scores for 4-year-old children were 80-85% (85.4% for European Spanish).

Though differences across languages, McLeod and Crowe (2018)McLeod, S., & Crowe, K. (2018). Children’s Consonant Acquisition in 27 Languages: A Cross-Linguistic Review. American Journal of Speech-Language Pathology, 27(4), 1546-1571. https://doi.org/10.1044/2018_AJSLP-17-0100
, after reviewing 64 studies involving more than 26,000 children and 27 languages concluded that 93% of consonants were correctly produced by 5 years old. In the same line, Stoel-Gammon (2006, p. 646)Stoel-Gammon, C. (2006). Infancy: phonological development. Encyclopaedia of Language & Linguistics (Second Edition), 642-648. https://doi.org/10.1016/B0-08-044854-2/00838-5
stated that “By the age of 3 years, the level of intelligibility increases to 75%, and by age 4, it is 100%”, meaning that, though not adult-like yet, the child phonological system is sufficiently developed to be intelligible.

In Spain, the theories set out first by Stampe and then by Ingram were later introduced by authors such as Bosch (1983)Bosch, L. (1983). El desarrollo fonológico infantil: una prueba para su evaluación. Anuario de Psicología, 28(1), 87-114.
and Díez-Itza (1995)Díez-Itza, E. (1995). Procesos fonológicos en la adquisición del español como lengua materna. In J. M. Ruiz, P. H. Sheerin, & E. González-Cascos (Eds.), Actas del XI Congreso Nacional de Lingüística Aplicada (pp. 225-264). Universidad de Valladolid.
. For both researchers, the phonological acquisition period is placed between approximately one and a half years old and six to seven years old, with an intermediate division around four years old (Bosch, 1983)Bosch, L. (1983). El desarrollo fonológico infantil: una prueba para su evaluación. Anuario de Psicología, 28(1), 87-114.
. This means that one cannot talk about a total control of the complete phonological system until the age of six or seven, when the child masters certain complicated phonemes and their combination in more complex syllables. In spite of that, as mentioned before (Bernhardt and Stemberger, 2017Bernhardt, B. M., & Stemberger, J. P. (2017). Investigating typical and protracted phonological development across languages. In E. Babatsouli, D. Ingram, & N. Müller (Eds.), Crosslinguistic Encounters in Language Acquisition: Typical and Atypical Development (pp. 71-108). Multilingual Matters. https://doi.org/10.21832/9781783099092-008
; Stoel-Gammon, 2006Stoel-Gammon, C. (2006). Infancy: phonological development. Encyclopaedia of Language & Linguistics (Second Edition), 642-648. https://doi.org/10.1016/B0-08-044854-2/00838-5
), by the age of 4 years intelligibility is complete.

Spanish studies have mostly focused on what Díez-Itza and Martínez López (2004)Díez-Itza, E., & Martínez López, V. (2004). Las etapas tardías de la adquisición fonológica: procesos de reducción de grupos consonánticos. Anuario de Psicología, 35, 177-202.
call periodo temprano ‘early period’, that is, until about three years old. These authors consider necessary to increase research on the periodo tardío ‘late period’, i.e., from three to six years old. They determined three stages in the phonological acquisition: expansión ‘expansion’, the stage until 3;0, characterised by a progressive diminution of phonological processes (such as unstressed syllable deletion, clusters reduction, etc.), after which there would be a standstill; estabilización ‘stabilisation’, from three to four years old and initially defined by a considerable decrease of processes, which increase again at around four years old (showing a U-shape developmental pattern); and resolución ‘resolution’ from the age of five years onwards, when phonological processes are residual. Díez-Itza and Martínez López’s (2004)Díez-Itza, E., & Martínez López, V. (2004). Las etapas tardías de la adquisición fonológica: procesos de reducción de grupos consonánticos. Anuario de Psicología, 35, 177-202.
intention was to confirm if the age of four clearly becomes a universal milestone of transition towards subsequent periods, as it has been repeatedly assumed by descriptive studies. In fact, at the age of four years children’s language is characterised, from the standpoint of phonology, by an increased speech rate, which means more coarticulation and the lengthening of utterances and conversational turns (Díez-Itza & Martínez López, 2004Díez-Itza, E., & Martínez López, V. (2004). Las etapas tardías de la adquisición fonológica: procesos de reducción de grupos consonánticos. Anuario de Psicología, 35, 177-202.
). Many scholars agree on a transition point at four years old (Bosch, 1983Bosch, L. (1983). El desarrollo fonológico infantil: una prueba para su evaluación. Anuario de Psicología, 28(1), 87-114.
; Díez-Itza & Martínez López, 2004Díez-Itza, E., & Martínez López, V. (2004). Las etapas tardías de la adquisición fonológica: procesos de reducción de grupos consonánticos. Anuario de Psicología, 35, 177-202.
) regarding phonological acquisition, but also other linguistic levels. For example, Maratsos (1974)Maratsos, M. P. (1974). Children who get worse at understanding the passive: A replication of Bever. Journal of Psycholinguistic Research, 3(1), 65-74. https://doi.org/10.1007/BF01067222
, analysing the acquisition of the passive structure, concluded that children show a U-shape developmental pattern around four years old, as the rate of passive comprehension decreased in comparison to younger children. Also, Garrote (2010)Garrote, M. (2010). Los corpus de habla infantil. Metodología y análisis. Servicio de publicaciones de la Universidad Autónoma de Madrid.
found that it was around 4;0 that children produced more non-targeted speech as a consequence of rule overgeneralisation errors.

Bosch (1983)Bosch, L. (1983). El desarrollo fonológico infantil: una prueba para su evaluación. Anuario de Psicología, 28(1), 87-114.
, based on studies by Serra (1983)Serra, M. (1983). Normas estadísticas de articulación para la población escolar de 3 a 7 años en el área metropolitana de Barcelona. Revista de Logopedia, Foniatría y Audiología, 3(4), 232-235. https://doi.org/10.1016/S0214-4603(83)75286-1
and Melgar de González (1976)Melgar de González, M. (1976). Cómo detectar al niño con problemas de habla. Trillas.
, summarised the most problematic phonemes during the acquisition process of Spanish: the trill /r/, fricatives such as /s/, /θ/ and /x/, and the voiced plosive /d/. She concludes that the most difficult place of articulation is that located in the dento-alveolar area, where a great number of sounds are differentiated just by the manner of articulation (Bosch, 1983)Bosch, L. (1983). El desarrollo fonológico infantil: una prueba para su evaluación. Anuario de Psicología, 28(1), 87-114.
. López Valero et al. (1989)López Valero, A., Carrillo Hernández, M. R., & Ros Frutos, J. L. (1989). Aportaciones para el estudio del desarrollo del lenguaje infantil en el período comprendido entre los veinticuatro y los treinta meses. Cauce, Revista de Filología y su Didáctica, 12, 145-156.
supported Bosch’s findings concluding that the sounds belatedly acquired in Spanish are /x/, /f/, /r/ and / θ /.

Other authors such as Serra (1983)Serra, M. (1983). Normas estadísticas de articulación para la población escolar de 3 a 7 años en el área metropolitana de Barcelona. Revista de Logopedia, Foniatría y Audiología, 3(4), 232-235. https://doi.org/10.1016/S0214-4603(83)75286-1
established the following order of acquisition: nasals, plosives, fricatives, and, finally, liquids and the alveolar trill.

It is noteworthy to mention here two studies related to the present one, due to the age range (3 to almost 6 years old) and the language (Spanish, though Mexican variety). First, Jiménez (1987)Jimenez, B. C. (1987). Acquisition of Spanish consonants in children aged 3-5 years, 7 months. Language, Speech, and Hearing Services in Schools, 18(4), 357-363. https://doi.org/10.1044/0161-1461.1804.357
found out that, by age 5 years, the 120 children forming the sample showed production problems only with two consonants: /s/ and /r/. Second, Acevedo (1993, p. 11)Acevedo, M. A. (1993). Development of Spanish consonants in preschool children. Communication Disorders Quarterly, 15(2), 9-15. https://doi.org/10.1177/152574019301500202
also tested 120 Mexican children. Results proved that sound “mastery occurred by the 4;0-4;5 age group”, remaining problematic the following consonants: /ɲ/, /g/, /f/, /s/, and /x/. Both studies were based on elicitation tasks, not on spontaneous speech.

Most significant works on the Spanish phonological acquisition, unlike the present study, are focused on the early period and they are crosslinguistic studies (Bosch & Sebastián-Gallés, 2001Bosch, L., & Sebastián-Gallés, N. (2001). Evidence of early language discrimination abilities in infants from bilingual environments. Infancy, 2, 29-49. https://doi.org/10.1207/S15327078IN0201_3
, 2003Bosch, L., & Sebastián-Gallés, N. (2003). Language experience and the perception of a voicing contrast in fricatives: infant and adult data. In M. J. Solé, D. Recasens, & J. Romero (Eds.), Proceedings of the Fifteenth International Congress of Phonetic Sciences (pp. 1987-1990). Universitat Autónoma de Barcelona.
; Bunta & Ingram, 2007Bunta, F., & Ingram, D. (2007). The acquisition of speech rhythm by bilingual Spanish-and English-speaking 4-and 5-year-old children. Journal of Speech, Language, and Hearing Research, 50(4), 999-1014. https://doi.org/10.1044/1092-4388(2007/070)
; Goldstein & Cintrón, 2001Goldstein, B., & Cintrón, P. (2001). An investigation of phonological skills in Puerto-Rican Spanish-speaking 2-year-olds. Clinical Linguistics and Phonetics, 15, 343-361. https://doi.org/10.1080/02699200010017814
; Kehoe & Lleó, 2003Lleó, C. (2003). Prosodic licensing of coda in the acquisition of Spanish. Probus, 15, 257-281. https://doi.org/10.1515/prbs.2003.010
, 2005Kehoe, M., & Lleó, C. (2005). The emergence of language specific rhythm in German-Spanish bilingual children. Arbeiten zur Mehrsprachigkeit: Working Papers in Multilingualism, 58. SFB 538.
; Kehoe, Lleó & Rakow, 2005Kehoe, M., & Lleó, C. (2005). The emergence of language specific rhythm in German-Spanish bilingual children. Arbeiten zur Mehrsprachigkeit: Working Papers in Multilingualism, 58. SFB 538.
; Lleó, 2002Lleó, C. (2002). The role of markedness in the acquisition of complex prosodic structures by German-Spanish Bilinguals. International Journal of Bilingualism, 6, 291-313. https://doi.org/10.1177/13670069020060030501
, 2003Lleó, C. (2003). Prosodic licensing of coda in the acquisition of Spanish. Probus, 15, 257-281. https://doi.org/10.1515/prbs.2003.010
, 2006Lleó, C. (2006). The acquisition of prosodic word structures in Spanish by monolingual and Spanish-German bilingual children. Language and Speech, 49, 205-229. https://doi.org/10.1177/00238309060490020401
). However, the interest here is in knowing how, once the Spanish phonemes are acquired (late period), the children’s phonological system becomes as stable as the adults’ observing the frequency of use.

Taking into account previous research and the above-mentioned claims (Acosta & Ramos, 1998Acosta, V., & Ramos, V. (1998). Estudio de los desórdenes del habla infantil desde la perspectiva de los procesos fonológicos. Revista de Logopedia, Fonatría y Audiología, 18, 124-142. https://doi.org/10.1016/S0214-4603(98)75683-9
; Grunwell, 1981Grunwell, P. (1981). The development of phonology: A descriptive profile. First language, 3, 161-191. https://doi.org/10.1177/014272378100200601
; MacWhinney, 1996MacWhinney, B. (1996). Computational analysis of interactions. In P. Fletcher, & B. MacWhinney (Eds.), The Handbook of child language (pp. 152-178). Blackwell. https://doi.org/10.1111/b.9780631203124.1996.00006.x
, among others), there is a need for a phonological frequency-based analysis of the linguistic performance of children aged 3;0 to 6;0 (late period), using a spontaneous speech corpus as a data source.

3. METHODOLOGY

3.1. The CHIEDE corpus

 ⌅

CHIEDE, a spontaneous child language corpus of Spanish, is made up of approximately 60,000 words. About a third of the corpus consists of child language and the remaining is CDS. The main feature of CHIEDE is the spontaneity of interactions. The corpus is made up of transcribed recordings of communicative situations in their natural context. The recordings were carried out in central Spain, where the linguistic variety is Peninsular Spanish, in a medium-sized town. The speakers are monolingual and belonging to middle socioeconomic status regarding their families’ income and occupation.

The corpus presents two types of interactions: spontaneous collective interactions, recorded at a daily activity in the classroom where the whole group of children and the teacher informally chatted; and dialogues, in which an adult talks with a single child. Figure 1 shows the corpus design⁴For further details, consult the web site http://www.lllf.uam.es/ESP/Chiede.html#:~:text=El%20Corpus%20de%20Habla%20Infantil,comunicativas%20en%20su%20contexto%20natural.. Children were grouped according to their year of birth.

Figure 1.  Corpus design.

CHIEDE contains 58,616 word tokens in 30 text files for a total of 7 hours and 53 minutes of recordings in 30 audio files from n=59 child participants. Table 1 presents figures regarding word tokens, number of utterances, word types and the token/type ratio by age group.

Table 1.  Corpus data: Word tokens, utterances and types by age group

Age group	Word tokens	Utterances	Types	Token /type ratio
3;0-3;12	5,628	4,909	985	5.7
4;0-4;12	6,787	5,092	1,155	5.8
5;0-5;12	9,004	5,443	1,450	6.2
Adults	37,197	20,876	2,910	12.7
Total	58,616	36,320

The fact that the corpus was going to be published required being extremely respectful and compliant with the current legal framework. Consequently, before recording, parents, teachers and participants were properly apprised and asked to sign an informed consent agreeing to participate in the research. Regarding ethical concerns, all names were anonymised and, on occasion, parts of the recordings were cut and discarded due to sensitive information the children gave about their private lives.

The device used to record the corpus was a Sony DAT (Digital Audio Tape), which allows for a digital recording with professional quality, with a Sony Stereo microphone placed in the most adequate spot to capture the sound. Even so, when recording ambient sound, a certain level of background noise is inevitable; it is impossible to obtain studio sound quality. For this reason, a sound editing software (Wavelab, https://www.steinberg.net/es/wavelab/) was used to improve the quality of the recordings.

The topics of conversation were varied, but all of them related to the children’s everyday lives: what they did yesterday or the previous weekend, describing their family, talking about their friends, their pets, or the things they like to do, etc.

Each recording is aligned with its corresponding orthographic transcription, including a header with metadata or sociolinguistic and contextual information. In addition to the audio and the text files, two other kind of files are included: those with the sound-text alignment by utterances and those in XML format with morphosyntactic annotation. The files are identified with a name where the age of the child participant is specified.

4. RESULTS

Results⁵Statistical analysis was carried out using the software IBM SPSS Statistics. are presented in four separate sections. In the first one a frequency-based phonological inventory is provided to address research questions 1 and 3. The next two sections offer data regarding variability between the three age groups. Finally, data from CHIEDE are corroborated by comparing results with three corpora from the CHILDES database.

4.2. Standard deviation analysis

 ⌅

So far, all data presented belong to the whole corpus divided into age groups. However, to calculate the standard deviation a sub-corpus was extracted in order to get a balance between the participants. As seen in Figure 1, representing the corpus design, CHIEDE is divided into two sub-corpora: collective interactions and dialogues. In the former communicative setting, the number of subjects is about twenty children (see Figure 1 for exact numbers), and the participation of all of them is not equal. When extracting the phonemes inventory for each of the participants, it was observed that while for some of them the number of words was very high -and therefore they presented a high frequency of phonemes- for others figures were considerably lower due to their moderate participation. Hence, a decision was made to use just the dialogues sub-corpus for this task as only one child participates in each interaction, so the number of conversational turns increases and therefore his/her production in terms of number of words enlarges. In addition, it was found that the number of words uttered by the children was similar in each dialogue (Table 5). A balance needed to compare data from different subjects was thus obtained.

Table 5.  Sample sizes per child group.

Age	Words
3;0-3;12-year-olds	4,021
4;0-4;12-year-olds	4,416
5;0-6;0-year-olds	4,119
Total	12,556

Thus, the total sample consists of 24 children, equally divided into three age groups -3;0-3;12, 4;0-4;12 and 5;0-6;0 years old- each one made up of eight children, four boys and four girls (see Figure 1). The relative frequency was calculated from the automatic count of the absolute frequency of the twenty-three Spanish phonemes, and then, the standard deviation across all children in each age group was computed. Table 6 presents the values for each age group.

Table 6.  Standard deviation from relative frequency of phoneme tokens by age group⁸As results belong to a sample, the symbol representing the mean is xˉ and the symbol representing the standard deviation is s (instead of µ for mean and σ for standard deviation, symbols conventionally used to describe a population)..

3;0-3;12-year-olds			4;0-4;12-year-olds			5;0-6;0-year-olds
Phonemes	xˉ	s	Phonemes	xˉ	s	Phonemes	xˉ	s
e	12.95	2.40	a	13.51	2.47	e	13.97	0.73
a	12.68	2.20	e	13.51	2.24	a	12.16	1.32
o	11.80	1.32	o	10.35	1.72	o	11.14	0.58
i	8.57	1.78	i	8.19	1.81	s	8.71	1.35
n	7.87	1.12	s	8.02	1.85	i	8.22	1.34
s	7.76	1.39	n	7.65	0.66	n	7.15	0.94
ɾ	4.27	0.84	ɾ	4.82	0.79	ɾ	4.69	0.95
u	4.25	1.12	k	4.15	1.25	l	4.61	0.67
l	4.05	0.97	l	4.13	0.93	k	4.12	0.61
t	4.05	1.02	u	3.95	0.93	m	3.71	0.86
k	3.90	1.05	t	3.83	0.89	t	3.65	0.42
m	3.57	1.16	m	3.50	0.99	u	3.55	0.65
p	3.18	0.78	p	3.03	0.66	d	3.20	0.69
d	2.54	0.19	d	3.01	0.81	p	3.11	0.79
b	2.41	0.93	b	2.48	0.70	b	2.45	0.49
ʎ	1.52	0.67	g	1.28	0.26	g	1.19	0.52
ɡ	1.51	0.41	ʎ	1.07	0.39	ʎ	1.08	0.52
x	0.72	0.37	x	0.99	0.41	θ	1.00	0.25
ʧ	0.63	0.31	θ	0.77	0.44	x	0.70	0.18
θ	0.60	0.42	ʧ	0.52	0.21	ʧ	0.52	0.16
r	0.57	0.21	r	0.51	0.31	r	0.48	0.25
f	0.33	0.24	ɲ	0.45	0.38	f	0.38	0.26
ɲ	0.27	0.14	f	0.31	0.11	ɲ	0.21	0.06

Noting the values, the deviation degree of each phoneme in relation to the mean is appreciable, especially for the figures corresponding to the 4;0-4;12 years old group (11 out of 23 phonemes), which show a higher fluctuation from the mean. On the contrary, the 5;0-6;0 years old group displays less variation, although it is notable salient in four cases: /f/, /g/, /p/ and /ɾ/. To appreciate the differences more clearly, these data have been transferred to boxplots (Figures 2, 3 and 4). For the last values, due to the low frequency of phonemes, differences are hardly substantial; but for higher values, the degree of variability is noticeable.

Figure 2.  Variability in 3-year-old group (3;0-3;12).

Figure 3.  Variability in 4-year-old group (4;0-4;12).

Figure 4.  Variability in 5-year-old group (5;0-6;0).

In the boxplots, the form of the median line shows three distinct blocks: after the first six most frequent phonemes (/e/, /a/, /o/, /i/, /n/, /s/) there is a marked drop, after which the values are kept within a stable range until a second drop in the last and least frequent ones (from /x/ in the 3;0-3;12 and 4;0-4;12 years old groups, and /θ/ in the 5;0-6;0 years old group). The highest frequency rates are distributed among seven phonemes: vowels /a/, /e/, /i/, and /o/, the nasal /n/, and the fricative /s/ (mean above 7, Table 6). Within the second block, we find plosives, the vowel /u/, the liquids /l/ and / ɾ /, and the nasal /m/. Finally, the last block (mean below 1, Table 6), in which the frequency of sounds is moderate, includes the rest of the fricatives, the trill /r/, and the nasal /ɲ/; here the degree of variability decreases due to the low frequency of use.

Despite the fact that the median line pattern is similar for the three charts, in the first two age groups there are more striking irregularities, while the last age group’s plot shows a softer median curve. In the latter case the degree of deviation is lower, showing more consistency.

Again, Friedman’s Two-Way Analysis of Variance by Ranks presents an asymptotic significance of p = 0.018, detailed by age groups as follows:

Table 7 shows significant differences between 5 and 4-year-olds and between 5 and 3-year-olds. However, between the 3 and the 4 years old groups there seems to be no significant difference, which means that in the oldest age group (5;0-6;0 years old) there is a stabilisation of the phonological system, since figures for standard deviation are lower (as can be seen in 8), given that fluctuation from the mean decreases. At ages 3;0-3;12 and 4;0-4;12 years the values present a higher variation, especially for the most frequent phonemes. However, from 5 years old these differences disappear and the figures are stabilised, decreasing the distance between the values and the mean, in contrast to the irregularities which the other two age groups show, especially the 4;0-4;12 years old group. Thus, the idea of a turning point at the age of four years in the process of phonological acquisition is reinforced: again, it seems that it is from that age when children’s language begins to approach adult use.

Table 7.  Friedman’s Two-Way Analysis of Variance by Ranks (standard deviation).

Pairwise Comparisons (df 2)
Sample 1/Sample 2	Test Statistic	Std. Error	Std. Test Statistic	Sig.
5/4-year-olds	.652	.295	2.212	.027
5/3-year-olds	.783	.295	2.654	.008
4/3-year-olds	.130	.295	.442	.658

4.3. U-shape development at four years old

 ⌅

Linked to the question about whether 4;0 is a turning point in the language acquisition process, and to the above data (standard deviation analysis), it is relevant to describe the finding of the greatest variability of 4-year-olds in the present study as a sign of a U-shaped (inverted in the chart) development pattern. Figure 5 shows how variability (based on standard deviation) is higher for 11 out of 23 phonemes (43.5%) in the 4;0 group: /e/, /a/, /o/, /s/, /i/, /l/, /p/, / θ /, /x/, /r/, and /f/. Therefore, it can be concluded that, at least in these 11 cases, a U-shape development pattern can be observed. This issue will be thoroughly discussed later.

Figure 5.  U-shape development at four years old.

5. DISCUSSION

Revisiting research questions in light of the results, major findings are summarised here. Regarding the first research question posed in the present study, it can be concluded that, according to the sample, the phonological Spanish system is essentially acquired (in terms of intelligibility) at the age of three years (as shown in Table 2). Acquisition is here understood as development, that is, as a process where phonemes are already organised into patterns (what Velleman and Vihman (2002)Velleman, S. L., & Vihman, M. M. (2002). Whole-Word Phonology and Templates. Language Speech and Hearing Services in Schools, 33(1), 9-23 https://doi.org/10.1044/0161-1461(2002/002)
call templates) typical of the final stages of development in children, showing that units are rooted. According to Velleman and Vihman (2002, p. 20)Velleman, S. L., & Vihman, M. M. (2002). Whole-Word Phonology and Templates. Language Speech and Hearing Services in Schools, 33(1), 9-23 https://doi.org/10.1044/0161-1461(2002/002)
, “templates serve as a stepping stone in the direction of the adult system, despite the decrease in accuracy that may temporarily result”. Vihman (2018, p. 38)Vihman, M., & Wauquier, S. (2018). Templates in child language. In M. Hickmann, E, Veneziano, & H. Jisa (Eds.), Sources of variation in first language acquisition: Languages, contexts, and learners (pp. 27-44). John Benjamins Publishing Company. https://doi.org/10.1075/tilar.22.02vih
also states that “template formation is neither the outcome of a pre-existing principle nor an end in itself, but instead a dynamic (and momentary) child response, in the early stages of acquisition, to the phonological and lexical challenges of the language”.

It is generally accepted in Spanish phonological acquisition research that the most problematic phonemes are liquid consonants, the fricatives /s/, /θ/ and /x/, the nasal /ɲ/, and the plosive /d/ (Acevedo, 1993Acevedo, M. A. (1993). Development of Spanish consonants in preschool children. Communication Disorders Quarterly, 15(2), 9-15. https://doi.org/10.1177/152574019301500202
; Bosch, 1983Bosch, L. (1983). El desarrollo fonológico infantil: una prueba para su evaluación. Anuario de Psicología, 28(1), 87-114.
, Jiménez, 1978Jimenez, B. C. (1987). Acquisition of Spanish consonants in children aged 3-5 years, 7 months. Language, Speech, and Hearing Services in Schools, 18(4), 357-363. https://doi.org/10.1044/0161-1461.1804.357
). However, after analysing these sounds in CHIEDE, it can be observed that both the fricative /s/ and the liquids /l/ and /ɾ/ are among the most frequent phonemes. CHIEDE’s participants showed no added difficulty in their use, indicating that, although they may be problematic phonemes at the time of their acquisition, from three years old onwards these three sounds do not present any difficulty for children with typical development; in fact, they are widely used. Regarding the rest of the phonemes which are considered problematic, it can be concluded that they are characterised by a lower use. The higher frequency of certain phonemes over others is a lexical matter: “Thus, when we examine the lexicon (words) of a language, not all sounds have an equal opportunity to appear in all positions.” (Bernstein-Ratner, 1994, p. 351Bernstein-Ratner, N. (1994). Phonological analysis of child speech. In J. L. Sokolov, & C.E. Snow (Eds.), Handbook of research in language development using CHILDES (pp. 324-372). Hillsdale, NJ: Lawrence Erlbaum Associates.
). Certain phonemes, such as /r/ or /ɲ/, are less frequent in the Spanish lexicon, and thus their frequency of use is low (as seen in frequency lists, Tables 2 and 8 ).

Results from the present study shed light on the existence of a turning point at four years old in the process of L1 acquisition (research question 2). On the one hand, figures on PMLU and SMLU (Table 4) indicate that from four to five years of age there is a significant increase towards adult language. Furthermore, standard deviation (Table 6) shows how language becomes stabilised from five years old onwards. It can also be stated that the subjects from this study fit Díez-Itza and Martínez López’s (2004)Díez-Itza, E., & Martínez López, V. (2004). Las etapas tardías de la adquisición fonológica: procesos de reducción de grupos consonánticos. Anuario de Psicología, 35, 177-202.
stages, as it seems that from 3;0 to 5;0 years old children are in a period of reorganisation of the phonological system, termed “stabilisation” by the authors; however, from 5;0 years old onwards children seem to achieve the “resolution” stage. Variability showed by the group of 4;0-4;12 leads to the conclusion that around four years old there is a landmark which is relevant not only for research on typical language development, but specially for research on speech and language disorders. This turning point is also supported by the U-shape development pattern evidenced from the analysis in Figure 5. Although the 3-year-olds group displayed a similar pattern, this was shown in those less frequent phonemes. However, 4-year-olds exhibited a higher variation and a U-shape pattern precisely for those phonemes which are acquired earlier and, therefore, should be stable at this age.

The overriding question guiding this research is to what extent the input frequency is relevant in the L1 acquisition process (research question 3). In disciplines such as Psycholinguistics, and more specifically in Speech and Language Therapy, it is quite accepted, that phonemes which usually pose a problem in the acquisition process, such as the Spanish trill, are characterised by a more difficult physiological articulation (Bosch, 1983Bosch, L. (1983). El desarrollo fonológico infantil: una prueba para su evaluación. Anuario de Psicología, 28(1), 87-114.
; López Valero et al., 1989López Valero, A., Carrillo Hernández, M. R., & Ros Frutos, J. L. (1989). Aportaciones para el estudio del desarrollo del lenguaje infantil en el período comprendido entre los veinticuatro y los treinta meses. Cauce, Revista de Filología y su Didáctica, 12, 145-156.
). However, this idea is conceived from the standpoint of adult speakers whose articulatory system is fossilised. The baby’s physiology is ready to adapt to different circumstances and therefore we cannot claim whether it is difficult for a child to manage his/her articulators to pronounce a sound or if he/she simply lacks enough examples to learn it. According to Zamuner et al. (2004, p. 1420)Zamuner, T., LouAnn, S., & Gerken y Hammond, M. (2004). Phonotactic probabilities in young children’s speech production. Journal of Child Language 31, 515-36. https://doi.org/10.1017/S0305000904006233
, “it appears that children are not limited by articulatory or perceptual constraints, but rather that children’s errors are largely influenced by their ability to access stored representations.”. For these reasons, and mainly based on the results obtained from CHIEDE, it is highlighted here the relevance of probability and frequency in studies on language ontogenesis, as frequency of use may be an essential indicator of typical development.

It is also agreed that at the age of three all vowels are acquired, followed by nasals, approximants, and later plosives. However, at this age, the incomplete acquisition of liquids, fricatives and affricates prevails (LLeó, 2012Lleó, C. (2012). First language acquisition of Spanish sounds and prosody. In J. I. Hualde, A. Olarrea, & E. O’Rourke (Eds.), The Handbook of Hispanic Linguistics (pp. 693-710). Blackwell Publishing Ltd. https://doi.org/10.1002/9781118228098.ch32
). Interestingly, this order of acquisition coincides with the order of frequency of spontaneous adult speech phonemes in Spanish (Table 2).

Studies such as those by Demuth (2009)Demuth, K. (2009). The prosody of syllables, words and morphemes. In E. L. Bavin (Ed.), The Cambridge Handbook of Child Language (pp. 183-198). Cambridge University Press. https://doi.org/10.1017/CBO9780511576164.011
, Ellis (2017)Ellis, N. C. (2017). Cognition, Corpora, and Computing: Triangulating Research in Usage-Based Language Learning. Language Learning, 67(S1), 40-65. https://doi.org/10.1111/lang.12215
, Kern et al. (2014)Kern, S., Gayraud, F., & Chenu, F. (2014). The role of input in early first language morphosyntactic development. Language, Interaction and Acquisition, 5(1), 1-18. https://doi.org/10.1075/lia.5.1.00int
or Tomasello (2009)Tomasello, M. (2003). Constructing a language: A usage-based theory of language acquisition. Harvard University Press.
, among several others, demonstrate the probabilistic relationship between input and language acquisition. The present study is another example of how the input frequency affects language development (in this particular case, phonological acquisition). “Ease of articulation seems to play only a partial role in determining the overall developmental route” (Pye, Ingram & List, 1987, p. 182Pye, C. L., Ingram, D., & List, H. (1987). A comparison of initial consonant acquisition in English and Quiché. In K. Nelson, & A. Van Kleeck (Eds.), Children’s Language (Vol. 6) (pp. 175-190). Erlbaum. https://doi.org/10.4324/9781315792668-8
).

Another factor influencing phonological learning is phonological neighbourhoods or phonologically similar words. Studies such as those by Zamuner (2009)Zamuner, T. (2009). The structure and nature of phonological neighbourhoods in children’s early lexicons. Journal of Child Language, 36, 3-21. https://doi.org/10.1017/S0305000908008829
showed that the words which are first acquired have denser neighbourhoods than those acquired later. Maekawa and Storkel (2006)Maekawa, J., & Storkel, H. L. (2006). Individual differences in the influence of phonological characteristics on expressive vocabulary development by young children. Journal of Child Language, 33, 439-459. https://doi.org/10.1017/S0305000906007458
also highlighted the importance of phonotactic probability and density neighbourhood. These authors concluded that “[...] phonotactic probability, density and frequency appeared to predict expressive vocabulary development but with individual variation across children” (Maekawa & Storkel, 2006, p. 457Maekawa, J., & Storkel, H. L. (2006). Individual differences in the influence of phonological characteristics on expressive vocabulary development by young children. Journal of Child Language, 33, 439-459. https://doi.org/10.1017/S0305000906007458
).

Likewise Pierrehumbert (2003)Pierrehumbert, J. B. (2003). Probabilistic theories of phonology. In R. Bod, J. Hay, & S. Jannedy (Eds.), Probability Theory in Linguistics (pp. 177-228). The MIT Press.
referred to various studies that have shown that children are sensitive to statistical patterns of sound. This stands in opposition to the idea of a universal inventory from which the individual selects the necessary elements to design his/her phonological system. The main counter-argument she stated is that this theory does not explain why children take so much time from when they acquire or distinguish an element as one of their own language until they master its production in an adult manner. Phonetic knowledge is gradually acquired and it is updated through experience. “Acquiring the phonetic encoding system of a language involves acquiring probability distributions over the phonetic space”⁹ Pierrehumbert (2003) defined the concept of phonetic space as the acoustic and articulatory parameterisation of speech as physical event, that is, what Moreno Cabrera (1997) called espacio de variación articulatorio (‘articulatory variation space’), or the different articulatory realisations of a phoneme. (Pierrehumbert, 2003, p. 184Pierrehumbert, J. B. (2003). Probabilistic theories of phonology. In R. Bod, J. Hay, & S. Jannedy (Eds.), Probability Theory in Linguistics (pp. 177-228). The MIT Press.
).

This last idea leads to consider how crucial the roles of probability and frequency of use are in the process of language acquisition. Bernstein Ratner (1994)Bernstein-Ratner, N. (1994). Phonological analysis of child speech. In J. L. Sokolov, & C.E. Snow (Eds.), Handbook of research in language development using CHILDES (pp. 324-372). Hillsdale, NJ: Lawrence Erlbaum Associates.
suggested that those elements that children acquire earlier are the most frequent both in adult speech and in all languages throughout the world, while phonemes that present a higher learning difficulty are precisely those that are less represented.

In this study (Table 2), the frequency of use that the oldest age group shows is very similar to that shown by adults in spontaneous speech, whereas the differences between the other two groups of children and the adult one are larger. According to data from CHIEDE, the most common sounds in adult speech are precisely those which, based on previous research (Bosch, 1993Bosch, L. (1983). El desarrollo fonológico infantil: una prueba para su evaluación. Anuario de Psicología, 28(1), 87-114.
; López Valero et al., 1989López Valero, A., Carrillo Hernández, M. R., & Ros Frutos, J. L. (1989). Aportaciones para el estudio del desarrollo del lenguaje infantil en el período comprendido entre los veinticuatro y los treinta meses. Cauce, Revista de Filología y su Didáctica, 12, 145-156.
; Serra, 1983Serra, M. (1983). Normas estadísticas de articulación para la población escolar de 3 a 7 años en el área metropolitana de Barcelona. Revista de Logopedia, Foniatría y Audiología, 3(4), 232-235. https://doi.org/10.1016/S0214-4603(83)75286-1
), are acquired earlier and more easily, i.e., vowels and nasals in the first place, followed by plosives and liquids. Lower positions on the frequency list are occupied by fricatives, which are precisely the last and most problematic in the acquisition process.

The same phenomenon occurs in other languages. For example, the English sounds identified as more complicated to learn (Grunwell, 1981Grunwell, P. (1981). The development of phonology: A descriptive profile. First language, 3, 161-191. https://doi.org/10.1177/014272378100200601
) are those which have a lower frequency rate in adult language (Mines et al., 1978Mines, M. A., Hanson, B. F., & Shoup, J. E. (1978). Frequency of occurrence of phonemes in conversational English. Language and speech, 21(3), 221-241. https://doi.org/10.1177/002383097802100302
). Among these phonemes are some fricatives, such as the voiceless dental /θ/ and the voiceless and voiced postalveolar affricates /ʧ/ and /ʤ/. There is an undeniable relationship between less frequent phonemes in adult language and those which are more problematic in the acquisition process.

The evidence so far leads to emphasize the importance of the input frequency in the study of L1 acquisition and its relation to the most problematic phonemes. With this, the importance of the place and manner of articulation as the sole factor causing the delayed acquisition of certain phonemes should be played down (Rose, 2009Rose, Y. (2009). Internal and External Influences on Child Language Productions. In Pellegrino, François, Egidio Marsico, Ioana Chitoran, & Christophe Coupé (Eds.), Approaches to Phonological Complexity (pp. 329-351). Mouton de Gruyter. https://doi.org/10.1515/9783110223958.329
). As stated by Menn and Stoel-Gammon (1996, p. 352)Menn, L., & Stoel-Gammon, C. (1996). Phonological development. In P. Fletcher, & B. MacWhinney (Eds.), The Handbook of child language (pp. 335-359). Blackwell. https://doi.org/10.1111/b.9780631203124.1996.00014.x
, “A theory of child phonology cannot ignore word frequency although current adult phonological theory has no place for this notion”.

6. CONCLUSIONS

Research on language acquisition beyond English and crosslinguistically has thrived during the last decades, although many unsolved questions still remain. There is a need for large cross-sectional spontaneous speech corpora, sufficiently representative and linguistically annotated. Furthermore, standards must be established to facilitate analysis and comparison. As Stoll (2009, p. 91)Stoll, S. (2009). Crosslinguistic approaches to language acquisition. In E. L. Bavin (Ed.), The Cambridge Handbook of Child Language (pp. 89-104). Cambridge University Press. https://doi.org/10.1017/CBO9780511576164.006
complained, “the use of different data sets, different methods or different criteria for coding makes it difficult to compare across languages”. Also, corpus-based analysis of the late acquisition period should be increased, that is, exceeding 36 months old, as most of the existing corpora do not include child participants exceeding that initial period of language development. The use of representative corpora and computational tools enriches research on language acquisition and is a reliable method for the study of frequency, which, as several investigations reveal, is a significant factor throughout the acquisition process.

The findings from the present study contribute to current research on Spanish-speaking children’s phonological acquisition in three ways:

From a methodological point of view, we encourage language acquisition research based on natural language corpora. Corpus Linguistics and Computational Linguistics are essential in language analysis, especially from a usage-based approach, as commented above and showed in this research. In addition, apart from the three contributions mentioned above, the findings of this research have practical implications for Clinical Linguistics and Speech and Language Therapy, as they can be used as a paradigm for the assessment of child language