Process Density Index (PDI) scores in typically developing children between 3;0 to 7;0 years of age: Evidence from Dravidian Tulu

Abstract

Various speech metrics have been developed to assess and monitor phonological development in young children. Process Density Index (PDI) is one such measure employed to determine the severity of phonological errors and speech intelligibility. The present study explored the utility of PDI as a measure of phonological development in 756 Tulu-speaking children between 3;0–7;0 years, across age and gender groups. Results revealed that the youngest group of children exhibited a higher number of phonological processes and a higher PDI score as compared to the oldest group. Children with higher PDI scores exhibited poorer speech intelligibility than those with lower PDI Scores. Further, a statistically significant gender difference with boys exhibiting higher PDI scores than girls with no interaction between age and gender was noted. The study finding provides an insight into the phonological aspects specific to Tulu language and suggests the utility of PDI as a potential clinical measure while assessing phonological issues.

Keywords

phonological development phonological process Process Density Index

I Introduction

Systematic simplification of adult words involving alteration of a class of sounds or a word structure is known as a ‘phonological process’ (Dunn and Davis, 1983). Since the 1970s, researchers have been using the phonological process to characterize development of speech in children (Dodd et al., 2003; Ingram, 1981). For instance, children may exhibit processes such as substitution (/toup/ for ‘soup’), assimilation (/tat/ for ‘tap’), syllable structure (/wawa/ for ‘water’) and cluster reduction process (/tain/ for ‘train’; Lowe, 1994). These phonological processes exhibited by children at an early age, occur less frequently as they acquire an adult-like sound system (Bernthal and Bankson, 1988; Edwards and Shriberg, 1983; Stampe, 1972).

Dodd et al. (2003) explored the phonological processes among British English-speaking children (n = 684, 3;0–6;11 years). The authors reported a decrease in error patterns with increase in age. By six years of age, more than 90% of children did not show any phonological processes. Among the processes noticed, voicing resolved by 3;0 years; stopping by 3;6 years; weak syllable deletion and fronting by 4;0 years. While de-affrication and cluster reduction resolved by 5;5 years, liquid, gliding persisted up to 6;0 years.

There are various procedures and measures to assess phonological processes in typical and disordered populations (Hodson, 1980; Ingram, 1981; Shriberg and Kwiatkowski, 1980; Weiner, 1979). The various speech metrics employed are: Percentage of Consonants Correct (PCC; Shriberg and Kwiatkowski, 1982), Percentage of Consonants Correct–Adjusted (Shriberg et al., 1997), Percentage of Consonants Correct Revised (PCC-R; Shriberg et al., 1997), Percentage of Vowels Correct (PVC; Shriberg, 1993), Percentage of Vowels Correct–Revised (PVC-R; Shriberg et al., 1997), Percentage of Phonemes Correct (PPC; Dollaghan et al., 1993), Percentage of Phonemes Correct–Revised (PPC-R; Shriberg et al., 1997), Process Density Index (PDI; Edwards, 1992) and Phonological Disorder Score index (PDS; Hodson and Paden, 1991). These speech metrics are used to analyse the speech by categorizing the correct and incorrect speech sound production, thereby diagnosing and monitoring the speech of an individual (Shriberg et al., 1986).

Process Density Index (PDI) is one of the useful speech metrics developed by Edwards (1992) to assess and monitor phonological development. The PDI is defined as the ‘mean number of phonological processes uttered per word in a speech sample’ (van Borsel and D’haeseleer, 2018). PDI is considered as a reverse measure of PCC. While PCC is used to calculate the number of consonants that children can produce accurately, PDI is used to determine the number of consonants and vowels produced incorrectly. PDI is also referred to as ‘processes per word’ (Khan, 2001) where it considers even a single incorrect sound (Wertzner et al., 2005). For example, if a child produced 12 number of phonological processes in 50 stimulus words, then PDI is 0.24 (number of phonological processes divided by number of words). The PDI can be measured with any type of speech sample and assessment methods. As phonological process descriptors are considered as valid ways to describe error patterns (Flipsen et al., 2005), researchers have employed PDI as a valid clinical measure to identify the severity of phonological errors, speech intelligibility and articulatory competence (Khan, 2001; Peña-Brooks and Hegde, 2007).

Wertzner et al. (2006) analysed 50 Brazilian children with phonological disorders aged between 4;0 to 12;0 years. They reported negative correlation between the PCC and the PDI indicating that when one measure increases, the other measure decreases during picture naming and imitation task. This high correlation between the two indices confirms that these two measures can be used to find out the severity of the phonological disorder (Wertzner, 2002; Wertzner et al., 2004). Similarly, Barrozo et al. (2016) studied PDI scores in 21 Brazilian Portuguese children with phonological disorders between 7;0 to 9;11 years. Among these children, higher PDI scores were noted for children with auditory processing disorders as compared to children without auditory processing disorders. The authors recommended PDI as one of the measures, which can characterize the occurrence of all phonological processes. On similar lines, Martins et al. (2016) reported PDI scores in European Portuguese and Brazilian Portuguese children with speech sound disorders.

van Borsel and D’haeseleer (2018) investigated PDI scores in typically developing Dutch children aged between 2;6 years and 5;6 years. They found a strong negative relationship between age and PDI scores; older the child, the lower their PDI scores. They further reported that males obtained higher PDI scores than female peers indicating that speech accuracy was achieved earlier in girls than in boys. The study findings supported the existing data that the accuracy of speech production increases as the child grows older. However, they also reported that there was no influence of the socioeconomic status, history of otitis media and sibling status on PDI scores.

Wertzner et al. (2007) compared the severity metrics and certain factors such as medical history and audiological measures in 4;0–11;0-year-old children with phonological disorders. They reported that higher the age, higher the PCC, and lower the PDI scores. On the other hand, delayed speech and language development attributed to lower PCC and higher PDI scores. Wertzner et al. (2013) found that children who presented with stop or fricative devoicing had more severe speech sound disorders and presented higher scores on PDI, number of omissions, distortions, and substitution. The finding supports the importance of quantitative measurement to assess the severity of the phonological disorder.

As each language is unique with a distinct set of phonemes and phonological structure, the findings in one language are not comparable directly with another language. Therefore, there is a dire need to focus on the phonological processes and the PDI, specific to the language of testing. Indian language studies examining phonological processes have revealed the presence of language-specific processes such as retroflex fronting, medial syllable deletion, final syllable deletion, trill substitution, trill replacement, retro flexing, cluster reduction, gemination and degemination (Kaur et al., 2013; Ramadevi, 2006; Ramadevi and Prema, 2002; Rupela and Manjula, 2006; Sreedevi and Shilpashree, 2008; Veena et al., 2015). Preliminary investigations done in 60 Tulu-speaking children between 3;0 to 4;0 years revealed that Tulu-speaking children exhibited phonological skills that were both comparable to and divergent from those exhibited in other languages. The prominent processes observed were retroflex fronting, cluster reduction, and trill deletion whereas prevocalic voicing and metathesis were the least occurring processes. The study findings also revealed that processes such as de-nasalization, epenthesis, velar assimilation, gliding, bilabial assimilation, and initial syllable deletion were present only between 3;0 to 3;6 years of age (Shruthi, 2010). Even though there are studies on phonological processes and its disorders, none has reported on PDI as a measurement of phonological processes in Indian languages. The present study focused on Tulu, a South Indian Dravidian language. Tulu Nadu is a region that includes Udupi, Dakshina Kannada, and Kasaragod Taluk, where, Tulu is the widely spoken language. Tuluva people are also found in Maharashtra, Bangalore, Southern parts of Kerala, and Commonwealth nations. In the Indian context, 1,846,427 people speak Tulu as their major language of communication (Census of India, 2011).

Though Tulu is confined to a small region of India, it possesses a very rich vocabulary and has been considered a highly developed language by linguists. There are 5 regional and 4 social dialects. Regional dialects are Southwest, Southeast, Southcentral, Northwest, and Northeast. The 4 social dialects are broadly classified as brahmin dialect, Jain dialect, common dialect, Harijan and tribal dialects (Kekunnaya, 1994). There are 14 vowels (7 short and 7 long vowels) and 34 consonants in Tulu. The aspirated and un-aspirated contrasts for all consonants are maintained mostly in informal speech with the contrast more consciously maintained in the brahmin dialect. Hence, the aspirated consonants freely vary with un-aspirated ones in informal speech of all dialects. There are five nasals in the language which varies across the dialects. Dialectal variations are noted for aspirated stop consonants, sibilants, alveolar and retroflex laterals. No consonant occurs in the final position (Kekunnaya, 1994). The clusters which are found in Tulu can be broadly classified as two consonant clusters, three consonant clusters, and four consonant clusters. The two consonant clusters that appear in Tulu dialects may be divided into initial clusters and medial clusters, according to the position that they occur (Kekunnaya, 1994).

Currently, Speech-Language Pathologists (SLP) predominantly use informal self-developed assessment tools to evaluate phonological skills in children. Furthermore, research suggests that PDI can be considered as a clinically potential measure to assess and monitor the phonological development of a child. Considering the possibility of cross-linguistic variations in PDI, the current study aims to capture PDI as a measure of phonological development in typically developing Tulu-speaking children across age and gender. It was hypothesized that both age and gender will have a significant effect on PDI scores.

II Method

This study was conducted in different schools in Udupi district, Karnataka state, South India. Permission was obtained from school authorities before test administration. Informed written consent was taken from the parents and caregivers for the selected participants. The researcher explained the study to the parents and caregivers, verbally face to face and provided a written participant information sheet which contained information regarding the study purpose and the study details. Verbal assent was obtained from the participant before starting the assessment.

1 Participants

Using a cross-sectional study design, the participants were selected through a convenient sampling of available children from Anganwadi (a type of rural child care center in India started by the Indian government in 1975 as part of the Integrated Child Development Services program), nursery schools and various preschool centers. Typically developing children who speak Tulu as a primary language with a language age of 3 to 7 years (evaluated using Assessment of Language Development; Lakkanna et al., 2008) and children developing normally in the areas of physical, intellectual, social, sensory and language development (based on researchers’ observation, teachers and parental report) were included in the study. All the participants were from the middle socioeconomic status (confirmed with Udai Pareek’s Scale; Pareek and Trivedi,1995). Only those participants who readily agreed for participation were considered for the study. A total of 756 participants were selected and were categorized into eight age groups with six months’ interval. Table 1 details the distribution of children at the defined age groups.

Table 1.

Demographic details of the study participants.

Age group	Age range (years)	Mean age (years)	Male	Female	Total
Group 1	3;0–3;5	3;15	48	61	109
Group 2	3;6–4;0	3;6	43	32	75
Group 3	4;1–4;5	4;19	47	43	90
Group 4	4;6–5;0	4;6	55	51	106
Group 5	5;1–5;5	5;23	47	54	101
Group 6	5;6–6;0	5;6	37	45	82
Group 7	6;1–6;5	6;23	46	36	82
Group 8	6;6–7;0	6;6	54	57	111
Total			377	379	756

2 Procedure

a Study tool

66 single word picture naming task was used for the present study (Supplemental Material – Appendix 1). The tool consisted of all the phonemes in all the positions along with 9 clusters in the Tulu language. The phonemes /sh/, /h/ and those with aspiration were excluded, considering the phonemic inventory of the language. The word list consisted of one to four-syllable length with the following syllable pattern: CV, VCV, VCCV, CVCV, CVCVCV, CVCVCVCV, CCVC, CVCCV, CVCCCV, VCCVCV, CVCCVC, CVCCVC, CVCCVCV, CCVCVC, and CVCCVCCV.

b Test environment

The data of each participant was obtained independently. The picture stimuli were administered in a quiet room, either in a vacant classroom or at home. The speech samples were audio-recorded with a Sony Digital Voice Editor 2.4, ICD-P Series (professional voice recorder) placed approximately three inches away from the speaker’s mouth. The test duration ranged from 15-20 minutes for 66 target words. Further, the recorded speech samples were uploaded in hard disk and computer for a detailed analysis.

c Instructions

The investigator gave instructions in the Tulu language. For each picture stimuli, the child was instructed to name the pictures by probing with certain wh-questions such as ‘what is this?’ (/eina undu?/) to obtain each target word. Absence of response or irrelevant response to the picture stimuli was facilitated with semantic cues, e.g. for the word /gejje/ (‘anklet’), the semantic cue given was ‘name the ornament that we wear on the leg?’ (/akka karugu padombina eina/?). If the child was not responding to the semantic cue, the child was then instructed to imitate the verbatim. Further, the investigator introduced the next stimulus. After obtaining responses for 5 picture stimuli consecutively, the picture stimulus which was imitated earlier was presented again to obtain the spontaneous response of the target word for the final analysis. Social and tangible reinforcement (praise, smile, and candy) were given after task completion.

3 Data analysis

The recorded speech sample of each child was transcribed separately by two qualified SLPs to identify the different phonological processes such as vowel processes, substitution processes, syllable structure processes, and assimilation processes. The phonological process was analysed for all 66 words elicited across age and gender. The error pattern analysis was employed as it provided information regarding the errors in different classes of sounds or sound sequences during the child’s production. This further provided the information regarding the child’s sound system in terms of the type and frequency of phonological processes (Goldstein and Iglesias, 2001; Lowe, 1994; Morrow et al., 2014).

Total Phonological Process (%) = \frac{Frequency of Phonological process}{Total Phonological process} \times 100

Process Density Index (PDI) was calculated for each child using the following equation.

PDI = \frac{Total number of phonological process}{Total number of t a r g e t words uttered}

4 Statistical analysis

Statistical analysis was done using SPSS version 17. Descriptive statistics in terms of frequency of phonological processes were done for each age group and gender. A two-way ANOVA was used to analyse the PDI scores across eight age groups and gender groups. The significant difference across the groups was calculated using p < 0.05 criteria at 95% confidence interval. Based on the analysis, appropriate inferences were drawn.

III Results

A total of 756 typically developing Tulu-speaking children participated in the study. The data findings are reported in terms of the number of phonological processes and the PDI scores across age and gender. The different types of phonological processes analysed in the present study are illustrated in Supplemental Material – Appendix 2.

1 Phonological processes across age

Figure 1 illustrates the presence of phonological processes across age groups. The phonological process at the age of 3;0 years was 24%, 12.16% at 5;0 years and 6.9% at 7;0 years, a reflection of its reduction with age. However, a noticeable decrease in the percentage of occurrence was noted from 3;6-4;0 years of age groups when compared to other age groups.

Figure 1.

Phonological processes across age.

2 Phonological processes across gender

Figure 2 illustrates the use of phonological processes across gender in each age group. Excluding Group 1, the percentage of the phonological process was noted to be lesser in females as compared to males. At the age of 3;0 years, the phonological process % scores were 45 and 54.68 in males and females respectively. At the age of 3;6 to 4;0 years, it was 60% and 39%; at the age of 4;1 to 4;5 years, 58.64% and 41.36 %; at the age of 4;6 to 5;0, 51% and 49% for male and females, respectively. Thereafter, at the age of 5;0 to 5;5 years, the percentage score for the phonological process was 52% and 47%, and at the age of 6;0–6;5 years it was 60% and 39% in males and females respectively. At the age of 7;0 years, the percentage scores for the phonological process were 51% and 48% in males and females respectively.

Figure 2.

Phonological processes across gender.

3 PDI scores across age and gender

Regarding the effect of age on the PDI scores, the descriptive statistics revealed that PDI mean scores decreased with age, ranging from 0.31 to 0.09 as indicated in Table 2. The analysis of variance showed that the effect of age on PDI scores was statistically significant, F(7, 748) = 58.917, p < 0.001. The post hoc analysis using Tukey’s-b for significance, indicated no statistically significant differences between the adjacent age groups (Groups 8 and 7, Groups 6 and 5, Groups 4 and 3, Groups 2 and 1) as indicated in Table 3.

Table 2.

Descriptive statistics of Process Density Index (PDI) scores across age and gender.

Group	Gender	Mean	Standard deviation	n
Group 1	Male	0.31	0.12	48
	Female	0.30	0.11	61
	Total	0.31	0.12	109
Group 2	Male	0.26	0.13	43
	Female	0.23	0.12	32
	Total	0.25	0.13	75
Group 3	Male	0.28	0.13	47
	Female	0.21	0.13	43
	Total	0.25	0.13	90
Group 4	Male	0.18	0.09	55
	Female	0.19	0.10	51
	Total	0.19	0.10	106
Group 5	Male	0.19	0.12	47
	Female	0.15	0.09	54
	Total	0.17	0.11	101
Group 6	Male	0.14	0.07	37
	Female	0.12	0.10	45
	Total	0.13	0.09	82
Group 7	Male	0.10	0.06	46
	Female	0.09	0.09	36
	Total	0.10	0.07	82
Group 8	Male	0.09	0.06	54
	Female	0.08	0.05	57
	Total	0.09	0.06	111

Table 3.

Post hoc analysis of Process Density Index (PDI) scores across age.

Group	n	Subset for alpha = 0.05
		1	2	3	4	5
Group 8	111	0.0861
Group 7	82	0.0961	0.0961
Group 6	82		0.1295
Group 5	101			0.1676
Group 4	106			0.1858
Group 3	90				0.2483
Group 2	75				0.2507
Group 1	109					0.3058

Gender comparison of overall PDI scores unveiled a statistically significant difference between PDI scores for males (Mean = 0.19, SD = 0.13) and for females (Mean = 0.17, SD = 0.13); t(754) = 2.35, p = .019. Within each age group, as indicated in Table 2, the boys showed higher PDI scores than girls (except for Group 4) and the mean scores decreased from 0.31 to 0.09 in males and 0.30 to 0.08 in females with an increase in age.

On the univariate analysis, with age groups and gender as the independent variables and the PDI scores as the dependent variable, the analysis showed an effect of gender on the PDI scores within each age group, F(1, .109) = 10.616, p = .001. However, there was no significant interaction between gender and age groups on the PDI scores, F(7, .075) = 1.036, p = .404.

IV Discussion

The present study focused on the proportion of phonological processes and PDI scores among Tulu-speaking children between 3;0 to 7;0 years across both gender groups. A decrease in the number of processes with an increase in age was observed leading to better accuracy in speech sound production and enhanced speech intelligibility. While the percentage of phonological processes was 24% by 3;0 years, it had decreased to 6.9% by 7;0 years. The presence of phonological processes in Group 8, indicated that the phonological development continues beyond the age of 7;0 years. This observation is in consonance with previous research findings (Dodd et al., 2003; Shriberg, 1993; Smit et al., 1990). A decrease in the number of phonological processes with an increase in age has also been reported in Indian languages such as Kannada, Malayalam, Tamil, Telugu, Punjabi, and Hindi (Anilsam, 1999; Bharathy, 2001; Jayashree, 1999; Kaur et al., 2013; Nishthar, 2005; Perumal et al., 2017; Pootheri, 1998; Radhakrishnan, 2001; Ramadevi, 2006; Ramadevi and Prema, 2002; Santhosh, 2001; Singh, 2005; Sunil, 2008; Venkatesh et al., 2010). However, in the present study, a notable decrease in the occurrence of phonological processes at 3;6 to 4;0 years could be attributed to reduced sample size (n = 32).

The study further explored the gender effects on phonological processes. Across gender, the percentage of phonological processes was less in girls except in the age range of 3;0 to 3;5 years suggesting a higher rate of phonological development in them. Similar findings have been reported by van Borsel and D’haeseleer (2018) in terms of the number of phonological processes in girls and boys.

As the age increased, the phonological processes decreased for both males and females, gradually till the age of 7;0 years; the percentage scores for both boys and girls were observed to be similar except for the age group of 6;0–6;5 years (Group 7). At the age of 6;0–6;5 years, the phonological processes in males increased while in females, it decreased. This is probably due to the variability in terms of sample size and responsive rate towards target words. Finally, the phonological processes decreased for both males and females by the age of 7;0 years, portraying that the phonological development is not completed in both the genders and as reported in the literature (van Borsel and D’haeseleer, 2018). It is evident from the present study that the pattern of phonological development is non-linear. However, there is a relation between the percentage of the phonological process, gender, and age.

The study findings on PDI scores across age, indicated that children at 3;0 years produced 0.31 processes per word which decreased to 0.09 by the age of 7;0 years. A similar trend of decrease in PDI scores with an increase in age was noted for both boys and girls, indicating improved speech intelligibility. This reduction of PDI scores across age and gender supports the recent findings among typically developing Dutch children (van Borsel and D’haeseleer, 2018). They reported a mean PDI score of 0.53–0.03 for Dutch-speaking children between aged 2;5 to 5;5 years of age. Mean PDI scores across gender ranged from 0.53 to 0.07 and 0.40 to 0.03 in males and females respectively. In support of the study hypothesis, the findings also indicated a statistically significant gender difference in PDI, with higher PDI scores among boys. This is similar to the findings reported in earlier studies (Shriberg, 1993; Smit et al., 1990; van Borsel and D’haeseleer, 2018).

A cross-linguistic comparison between the existing literature in Dutch and the present study in Tulu indicated a similar pattern in terms of reduction in PDI scores with phonological development in both males and females. In terms of the number of phonological processes and PDI scores, a faster phonological development was noted in Dutch as compared to Tulu. At the age of 5;5 years, while the number of phonological processes was 3.60 and the PDI score was 0.07 in Dutch, the number of phonological processes was 7.63 and the PDI score was 0.19 in Tulu-speaking children. While a PDI score of .26 ± .27 for boys and .15 ± .19 for girls was reported in the Dutch population, in the present study, a score of .19 ± .13 for boys and .17 ± .13 for girls was noted for Tulu-speaking population. The differences in response across languages could be attributed to the methodological variations, variability in response towards the target stimulus provided, difference in overall sample size, and gender distribution. The present study has certain limitations that needs to addressed. Though PCC is a widely used metric, the present study fails to explore the correlation of the PDI findings with PCC scores. Further, association with perceptual judgment of intelligibility was not included in the study. In terms of coding, though two SLPs transcribed the samples and identified the error types, both interrater and intrarater reliability analysis of the samples were not done.

V Conclusions

Overall, the study findings revealed a decrease in PDI scores and the number of phonological processes with an increase in age from 3;0 to 7;0 years in both gender groups. In comparison to other severity indices, the PDI scores can be analysed on any type of speech sample including single word picture naming task. PDI is, therefore, a potential clinical measure of phonological errors, instrumental in assessing the phonological deficits and speech intelligibility. The categorization based on severity serves as an important factor in clinical decision making. The study findings further provided an insight into the phonological aspects specific to Tulu language and its phonologic features, which would help in enhancing the efficiency in the diagnosis of phonological deficits in Tulu-speaking children. Therefore, PDI can be readily used to supplement informal assessment and contribute towards limited normative data while assessing phonological issues in Tulu-speaking children. Such a measure could also be used to assess the effectiveness of language intervention and progress made by the child. The study findings also pave the way for further cross-linguistic research on the PDI metric including other Dravidian languages.

Supplemental Material

Appendix_1-_stimuli_list_Veena_K_D_MAHE-MCHP – Supplemental material for Process Density Index (PDI) scores in typically developing children between 3;0 to 7;0 years of age: Evidence from Dravidian Tulu

Supplemental material, Appendix_1-_stimuli_list_Veena_K_D_MAHE-MCHP for Process Density Index (PDI) scores in typically developing children between 3;0 to 7;0 years of age: Evidence from Dravidian Tulu by Sunila John, Kadiyali D Veena, Hanna Nelson, Venkataraja Udupi Aithal, Rekha Patil and Bellur Rajashekhar in Child Language Teaching and Therapy

Supplemental Material

Appendix_2-_Phonological_processP – Supplemental material for Process Density Index (PDI) scores in typically developing children between 3;0 to 7;0 years of age: Evidence from Dravidian Tulu

Supplemental material, Appendix_2-_Phonological_processP for Process Density Index (PDI) scores in typically developing children between 3;0 to 7;0 years of age: Evidence from Dravidian Tulu by Sunila John, Kadiyali D Veena, Hanna Nelson, Venkataraja Udupi Aithal, Rekha Patil and Bellur Rajashekhar in Child Language Teaching and Therapy

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Indian Council of Social Science Research (ICSSR), New Delhi

ORCID iDs

Kadiyali D Veena

Venkataraja Udupi Aithal

Supplemental material

Supplemental material for this article is available online.

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