Research in Developmental Disabilities 33 (2012) 737–747

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Research in Developmental Disabilities

Review article

Reading skills in children with Down syndrome: A meta-analytic review

Kari-Anne B. Næss a,*, Monica Melby-Lervag a, Charles Hulme a,b, Solveig-Alma Halaas Lyster a

a University of Oslo, Norwayb University of York, UK

A R T I C L E I N F O

Article history:

Received 11 September 2011

Received in revised form 19 September 2011

Accepted 21 September 2011

Available online 23 November 2011

Keywords:

Down syndrome

Reading

Literacy

Decoding

Meta-analysis

A B S T R A C T

The authors examine the reading profile in children with Down syndrome by comparing

the nonword decoding skills in children with Down syndrome and typically developing

children matched for word recognition level. Journal articles published before 04.05.2010

were identified by using the keyword Down* cross-referenced to ‘reading’, ‘literacy’,

‘decoding’, and ‘reading comprehension’ were selected. A total of eight papers met the

criteria for inclusion. Each study was reviewed and coded on both inclusion criteria and

coding protocol before the analysis was performed. Children with Down syndrome had

equivalent nonword decoding skills to typically developing children matched for word

recognition level, but showed deficits on measures of two important underlying skills,

vocabulary and phonological awareness. Differences in vocabulary, but not phonological

awareness, were predictive of differences in nonword decoding skills. The practical and

theoretical implications of these findings are discussed.

� 2011 Elsevier Ltd. All rights reserved.

Contents

1. Reading skills in children with Down syndrome: A meta-analytic review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738

2. Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739

2.1. Inclusion criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739

2.2. Data collection and identification of studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740

2.3. Screening of abstracts and full papers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740

2.4. Coding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740

2.5. Meta-analysis plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740

2.6. Homogeneity and moderators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740

2.7. Retrieval bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741

3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741

3.1. Nonword decoding skills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741

3.2. The influence of vocabulary, phonological awareness, and age on nonword decoding . . . . . . . . . . . . . . . . . . . . . . . 741

4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744

4.1. Implications for practise and future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744

5. Summary and conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746

* Corresponding author at: Department of Special Needs Education, University of Oslo, Postboks 1140, Blindern, 0318 Oslo, Norway. Tel.: +47 922 40 741.

E-mail address: k.a.b.nass@uv.uio.no (K.B. Næss).

0891-4222/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ridd.2011.09.019

K.-A. Næss et al. / Research in Developmental Disabilities 33 (2012) 737–747738

1. Reading skills in children with Down syndrome: A meta-analytic review

Learning to read is critical for educational success and effective independent living. There are limited data on readingabilities among children with Down syndrome, and the present study is the first meta-analytic review to investigatereading skills in children with this diagnosis. The aim of the review is to examine the profile of reading skills in childrenwith Down syndrome by looking at strengths and weaknesses compared with those of typically developing childrenmatched on word recognition level. The impact of vocabulary and phonological awareness on reading will also beassessed.

The overall goal of reading is to extract meaning from written text. To achieve this, two basic skills are required: decodingand language comprehension (Gough & Tunmer, 1986; Hoover & Gough, 1990; Stanovich, 1980). Decoding is the process oftransforming print into speech and depends on the child’s understanding and automatisation of the alphabetic principle.Decoding is usually measured by giving the child lists of words or nonwords to read aloud. Nonword reading is generallyseen as a pure measure of decoding skill: because nonwords have never been encountered before, the child has to find a wayto decode them either by applying sequential letter–sound decoding rules or by making analogies between the printednonword and similar known words. Decoding of nonwords therefore assesses the child’s ability to translate graphemes intotheir corresponding phonemes and blend them together.

Word recognition, on the other hand, may place some reliance on grapheme–phoneme decoding skills, but also taps intoother skills, with an increasing reliance on the retrieval of orthographic structures and pronunciations of familiar forms fromlexical memory. With practice, the recognition of familiar words becomes more automatic and accurate. Effective readingcomprehension depends upon a reader having efficient highly automatised word recognition skills. If readers haveinefficient non-automatised word recognition skills, fewer resources will be available for the higher level processes that arerequired for reading comprehension.

Several studies have found evidence that, with development, there is a shift in the relative importance of wordrecognition and language comprehension as predictors of individual differences in reading comprehension. In the earlyschool years, before a child’s word recognition skills are automatised, decoding skills explain much of the variance in readingcomprehension skills. As a child’s word recognition skills become automatic and fluent, variations in languagecomprehension gradually come to explain an increasing proportion of the variance in reading comprehension skills(Biemiller, 2005; Cunningham & Stanovich, 1997; Elleman, Lindo, Morphy, & Compton, 2009; Hagtvet et al., 2011; NationalReading Panel, 2000).

Several studies have shown that children with Down syndrome typically have severe limitations in languagecomprehension skills compared to typically developing children of the same nonverbal mental age (for a review see Næss,Lyster, Hulme, & Melby-Lervag, in press). Presumably, language deficits will also affect their phonological skills, sincevocabulary is often seen as an important prerequisite for the development of phonological awareness (Metsala & Walley,1998) and subsequent decoding skills (Kay-Raining Bird, Cleave, & McConnell, 2000). If it is true, as has sometimes beensuggested, that word recognition skills are a relative strength in children with Down syndrome and that these childrentherefore have better ‘technical reading’ skills than expected for their nonverbal mental age (e.g., Snowling, Nash, &Henderson, 2008), eventually problems with reading comprehension will be related to language deficits, and not to poordecoding skills.

Relatively few studies have examined decoding skills in children with Down syndrome. One reason for this may be that inthe past individuals with Down syndrome or other types of learning disabilities were not usually taught to read because oflow expectation concerning their developmental potential (Laing, 2002). However, children with Down syndrome are nowusually included in mainstream schools and taught to read. It is therefore important to get a better knowledge of theirreading profile, which in turn may lead to more effective methods for teaching these children to read.

Other factors that may explain the limited amount of research on reading are challenges concerning methodologicalissues in Down syndrome research such as, for example, matching procedures and difficulties of measurement (includingfloor effects). A few studies have used chronological age match design (e.g., Appelton, Buckley, & MacDonald, 2002; vanBysterveldt, Gillon, & Moran, 2006) and only a very small number of studies have directly compared reading skills inchildren with Down syndrome and typically developing children of the same mental (e.g., Cardoso-Martins, Peterson,Olson, & Pennington, 2009) or nonverbal mental age (e.g., Boudreau, 2002). In the existing studies, the use of a readinglevel matched design is most common (e.g., Baylis, 2005; Cossu, Rossini, & Marshall, 1993; Gombert, 2002; Hulme et al.,under review; Kay-Raining Bird, Cleave, Pike, & Helmkay, 2008; Snowling, Hulme, & Mercer, 2002; Verucci, Menghini, &Vicari, 2006; Roch & Jarrold, 2008). This means that children with Down syndrome are usually matched with youngertypically developing children at the same word recognition level, but with a higher mental ability. Although reading andlanguage skills are delayed in children with Down syndrome, visual word recognition skills are, as mentioned, often seenas a relative strength. Some studies have suggested that these children place a heavy reliance on a visual approach toreading single words, while phonological difficulties make the recognition of unfamiliar words (or decoding of nonwords)a relative weakness (Snowling et al., 2002). Results from a small scale study by Roch and Jarrold (2008) support this claim.They found that children with Down syndrome showed better word recognition than nonword decoding skills; they readin line with typically developing children matched on word recognition level when reading regular words. In contrast,children with Down syndrome scored poorly on measures of nonword decoding that required a phonological readingstrategy. Verucci et al. (2006) also found impaired nonword decoding in children with Down syndrome when this was

K.-A. Næss et al. / Research in Developmental Disabilities 33 (2012) 737–747 739

measured on a timed test, and suggest that correct decoding is only partially influenced by lexical access or semanticcontext. In contrast, Snowling et al. (2002) who used nonwords, graded in difficulty, placed on separate cards without anytime factor, found no differences in nonword decoding between a group of 29 children with Down syndrome and areading-level matched group (on word recognition) of 31 typically developing children. These results may indicate aweakness in the speed of phonological processing rather than a weakness in decoding skills. However, Hulme et al. (underreview), using a larger sample, found weaker untimed nonword decoding than word recognition in a study of 49 childrenwith Down syndrome and 61 typically developing control children roughly matched for initial levels of word recognitionskill.

In studies of typically developing children, the influence of different underlying cognitive processes on readingdevelopment have been examined extensively, and phonological awareness is seen as one of the important predictors. Instudies of children with Down syndrome there has been debate as to whether phonological skills are less important forlearning to decode than in typically developing children. Cossu et al. (1993) concluded that decoding skills in childrenwith Down syndrome did not rely on phonological skills at all. However, their conclusions have been criticised severelyby, e.g., Morton and Frith (1993) and Byrne (1993). These criticisms centred on the fact that phonological skills were notmeasured prior to learning to read, and also because the ‘meta-cognitive’ demands of the tasks used to assessphonological awareness (phoneme counting, phoneme deletion, oral spelling, and sound blending) may have beenbeyond the abilities of the children tested. In contrast to Cossu et al. (1993), Gombert (2002) found a clear link betweenphonological awareness and decoding skills in an experiment with 11 children with Down syndrome, mean age 13 years,9 months.

The study by Snowling et al. (2002) described earlier found a relationship between decoding and phonological skills aswell. In their study, children with Down syndrome did not differ from typically developing controls in the ability to detectphonemes in words when controlled for receptive vocabulary, but they had a deficit in rhyme detection awareness.Letter–sound knowledge did not predict decoding as it did for the typically developing controls. The authors thereforeconcluded that decoding in children with Down syndrome may less rely on phonological skills than it does in controls.However, Lemons and Fuchs (2010) showed in their systematic narrative review of 20 studies (with different designs)that decoding of words in children with Down syndrome, as in typically developing children, is related to variations inphonological skills.

The variation in results between studies and the relative impact of underlying skills as predictors of reading may reflectdifferences in age and developmental level. Bochner, Outhred, and Pieterse (2001), for example, found evidence for a positiverelationship between age, attendance in school, and decoding skills in 30 young adults with Down syndrome. In contrast,Groen, Laws, Nation, and Bishop (2006), in a traditional narrative review of 10 studies that reported word recognition skills inchildren with Down syndrome, found no relationship between age and decoding level, even though the age varied betweenthe studies from a mean CA of 95–252 months.

Overall, evidence seems to suggest that children with Down syndrome have a relative strength in word recognition inrelation to the difficulties they have decoding nonwords. This profile has been used as evidence for use of a logographic(whole word) reading strategy (Byrne, MacDonald, & Buckley, 2002; Kay-Raining Bird et al., 2000), which could be seen as avery first step in reading development. Since this strategy does not involve grapheme–phoneme decoding, it does notsupport the decoding of unknown words. In addition to children’s developmental level and the different kinds ofmeasurement used in different studies, variability in results may also reflect the small sample sizes in most studies. Smallsamples reduce statistical power, and hypotheses in a single study may be rejected or supported on a false basis. In contrast,the meta-analytic approach used in the present study allows us to synthesise the pattern of findings, by combining effectsizes from different studies using comparable measures, and weighting these effect sizes to take differences in sample sizeinto account.

The primary objective of the present study was to explore the reading profile of children with Down syndrome andinvestigate whether they have relatively more severe difficulties in nonword decoding than word recognition compared tothe younger word recognition control group. We expect phonological awareness to be weaker in children with Downsyndrome than in the controls, and we expect that variation in this variable will explain the variation in nonword decodingskills seen in children with Down syndrome as it usually does in studies of typically developing children. Receptivevocabulary skills and phonological awareness as moderators of nonword decoding will also be assessed.

2. Method

2.1. Inclusion criteria

We used the following criteria to select studies for inclusion in the meta-analysis:

1. W

e only included studies that compare children and adolescents with Down syndrome to typically developing childrenmatched on level of word recognition. Thus, we excluded studies using a reading comprehension measure or a mix ofnonword decoding and word recognition as the initial matching variable.

2. C

hildren and adolescents with Down syndrome had to be monolingual and had no additional specified diagnosis. Studiesof both children and adolescents with Down syndrome were included.

K.-A. Næss et al. / Research in Developmental Disabilities 33 (2012) 737–747740

3. T

he studies needed to report inferential statistics or means, standard deviations (or standard errors), and informationabout sample size for both groups.

2.2. Data collection and identification of studies

Papers published in English before 4 May 2010 were identified through electronic searches in PsycINFO and Eric (Ovid,OCLC, and CSA). The combinations of keywords related to Down* were crossed with ‘reading’, ‘literacy’, ‘decoding’, and‘reading comprehension’. We also wanted to include ongoing and unpublished studies in order to control for any systematicdifferences between published studies and grey literature (Rothstein, Sutton, & Borenstein, 2005). Well-known authors inthe field (e.g., Laws, Chapman, Kay-Raining Bird, and Buckley) were therefore contacted by e-mail to provide details ofongoing and unpublished studies. Also, the names of authors who generally publish in the field were used as keywords insearches in electronic databases (e.g., Buckley, Hulme, Kay-Raining Bird, and Snowling). In addition, reference lists of articlesalready included were assessed to identify additional studies, and a journal that specialises in publishing research on Downsyndrome was manually searched (Down Syndrome Research and Practise).

2.3. Screening of abstracts and full papers

All identified abstracts were reviewed twice according to the inclusion criteria by the first author. The coding wasconducted independently by the first author and one of the co-authors and results were then compared; any disagreementswere discussed in order to achieve agreement. Where the abstract gave insufficient information to exclude or include anarticle, the rest of the article was consulted in order to make the decision.

2.4. Coding

For each sample included in the meta-analysis, we coded the number of participants with Down syndrome and thenumber of typically developing children, in addition to inferential statistics or statistics reporting means and standarddeviations for measures of word and nonword decoding. Tests mainly drawing on recognition of single words were codedbefore the accuracy of word identification in texts. This was done in order to make the measures included as similar aspossible. When both reading speed and accuracy were reported, accuracy was chosen, and when more than one measureof word recognition accuracy was reported, a composite score was created. The following possible moderator variableswere coded: (a) letter knowledge, (b) vocabulary, and (c) phonological awareness. In order to increase the power of theanalyses and include the moderator analysis for all studies, a composite score of all the different measures ofphonological awareness was computed. If it is correct, as Snowling et al. (2002) have claimed, that children with Downsyndrome have more difficulty with some phonological awareness tasks than others, we also wanted to be as specific aspossible due to phonological awareness, and therefore (d) phoneme deletion, and (e) rhyme detection were also codedseparately.

2.5. Meta-analysis plan

We undertook a series of meta-analyses using the programme Comprehensive Meta-Analysis (Borenstein, Hedges,Higgins, & Rothstein, 2005). The effect sizes for each independent group comparison were calculated using Hedges’s g

(Hedges & Olkin, 1985). Hedges’s formula was used because it is corrected for sample size, and therefore does not tend tobe upwardly biased with small samples, as other effect size estimates tend to be. Hedge’s g is similar to Cohen’s d, andexpresses the magnitude of group differences in standard deviation units. In general, Hedge’s g = 0.2 can be interpreted asa small effect size, g = 0.5 as moderate and g = 0.8 as large (Cohen, 1988). When Hedges’s g is positive, the children withDown syndrome have the higher mean, and when it is negative the group difference is in favour of the typicallydeveloping children. Overall effect sizes were estimated by calculating a weighted average of the effect sizes from thestudies based on a random-effects model. The random-effects model rests on the assumption that variation in effect sizesacross studies is not only due to random error, but also reflects true variance caused by real differences between thestudies. For studies that involve differences in the measures used, as well as the age and background of participants, therandom-effects model appears most plausible (Borenstein, Hedges, Higgins, & Rothstein, 2009). For each effect size wereport a 95% confidence interval. When the confidence interval does not include zero, an effect size is statisticallysignificant at the .05 level.

2.6. Homogeneity and moderators

Under the random-effects model, we allow the true effect size to vary from study to study. In order to examine thevariation in effect sizes between studies, we computed the Q-test of homogeneity (Hedges & Olkin, 1985). Q is on astandardised scale and depends strongly on the number of studies. Because Q is a standardised measure, the expected valuedoes not depend on the metric of effect size. A significant value on this test indicates significant variability between the effectsizes in the sample of studies.

K.-A. Næss et al. / Research in Developmental Disabilities 33 (2012) 737–747 741

To examine the variation between studies, I2 was calculated. I2 is the percentage of total variation between the effect sizescaused by real heterogeneity rather than chance (Borenstein et al., 2009).

Since one aim of the study was to determine whether systematic differences between the studies can explain variations ineffect sizes, meta-regression based on the methods of moments was used to examine the impact of the continuousmoderator variables on the differences in effect sizes between studies (Borenstein et al., 2009). In order to determine thestrength of the predictors on study outcome, the proportion of between-study variance explained (R2) was used as an effectsize. The meta-regression was calculated using macros developed for SPSS (Lipsey & Wilson, 2001; Wilson, 2006). To avoidlack of power in the detection of meaningful differences across groups, we interpreted a value for moderator analyses onlywhen at least four studies reported the actual moderator variable (Bar-Haim, Lamy, Pergamin, Bakermans-Kranenburg, &van IJzendoorn, 2007).

2.7. Retrieval bias

In order to detect publication bias, funnel plots were examined for all analyses. Funnel plots are a graphic means forexamining the evidence for retrieval bias in a meta-analysis; a sample size-related measure (in this case, standard error) isplotted on the y-axis and the effect size on the x-axis. In the absence of publication bias, this plot should form a symmetricalinverted funnel (Borenstein et al., 2009). The trim and fill method was used in order to examine the impact from possiblemissing studies (Duval & Tweedie, 2000). The trim and fill method imputes values in the funnel plot to make it symmetrical,and estimates an adjusted overall effect size on this basis.

3. Results

The searches yielded a total of 781 publications, and the final set of unique studies after excluding duplicates was 434. Allabstracts were reviewed according to the inclusion criteria and, finally, after coding all the data and removing duplicates, atotal of eight papers met the criteria for inclusion. Characteristics of the included studies are presented in Table A1 in theAppendix.

The main reason for excluding an empirical study was that the study did not include a comparison group of typicallydeveloping children. Table 1 gives an overview of the search process. The studies by Cardoso-Martins and Frith (2001) andCardoso-Martins, Michalick, and Pollo (2002) included a group of children with Down syndrome matched to a group oftypically developing children matched on reading ability. But the initial matching measure also included nonwords, andthose studies were therefore excluded in order to match the children on the same variable we wanted to investigate.

Inter-coder reliability was estimated for 100% of the studies (for the continuous variables) as follows: for nonwordreading agreement rate = 100%, for word reading agreement rate = 100%, for phonological awareness agreement rate = 87.5%,for phoneme deletion agreement rate = 100%, for rhyme detection agreement rate = 100%, for vocabulary agreementrate = 87.5%. Disagreement was resolved by consulting the original paper and by discussion between the two coders.

3.1. Nonword decoding skills

A total of eight independent effect sizes compared 166 children with Down syndrome (mean sample size = 20.75,SD = 12.96, range 10–49) and 180 reading-level matched controls (mean sample size = 22.5, SD = 12.96, range 10–61) onnonword decoding. The effect size of each study and the overall mean effect size are shown in a Forest plot in Fig. 1. In theForest plot, studies are represented by symbols whose area is proportional to the study’s weight in the analysis and it showsan estimate of the study’s effect size, along with the 95% CI of the effect size, to provide a sense of the study-to-studydispersion.

The overall mean effect size of the eight studies (g = �0.63, CI [�1.35, 0.08], p = .08) showed that children with Downsyndrome had weaker performance on nonword decoding than typically developing children on the same word recognitionlevel, but the differences between the groups were not significant. The heterogeneity between the studies was significant,Q(7) = 65.01, p < .001 and the proportion of variation between the effect sizes attributable to real heterogeneity wasI2 = 89.23. The Forest plot in Fig. 1 indicates that, in the study by Baylis (2005), children with Down syndrome had a very lowscore on nonword decoding. However, if we exclude the Baylis’ study (2005), the result of the analysis is still not significant(g = �0.43, CI [�1.18, 0.30], p = .25), but the overall mean effect size is reduced, bringing the children with Down syndromecloser to the control group’s performance.

As for publication bias, a funnel plot indicated symmetry (no bias), and a trim and fill analysis added no studies.

3.2. The influence of vocabulary, phonological awareness, and age on nonword decoding

To explore potential sources of the heterogeneity in effect sizes between studies, we planned to code seven differentpotential moderators of outcome (differences in phonological awareness [both as a composite score and as individual scoreof phoneme deletion and rhyme detection], letter knowledge, RAN, vocabulary, and age). Unfortunately, no study reportedRAN, and only three studies reported letter knowledge, so these two moderator analyses could not be performed because toofew studies were available.

Stud y name

Baylis (200 5)Hulme et al. (su bmitted)Verucci et al. (200 6)Kay-Raining B ird et al. (200 8)Snowli ng et al. (200 2)Gombert (2002)Cossu et al. (199 3)Roch & Jarrold (200 8)

-2.00 -1 .00 0.0 0 1.0 0 2.0 0

Favours TD Favour s DS

Fig. 1. Overall mean effect size for nonword decoding (displayed by ^) and effect size with confidence interval for each study comparing children with

Down syndrome and reading matched controls.

Table 1

An overview of the results from the searching process.

(On the basis of www.pr ism a-statement.org )

Sear

chSc

reen

ing

Elig

ibili

tyIn

clud

ed

Search features:• Electron ic databases (n = 78 1)

- Eric (Ovid, OCLC and CSA) - PsychInf o, PubMed)

• Hand search (n = 100 ) - Journals that specialise in Down syndrome- Situation search- Scann ing of reference lis ts- Contact with autho rs

Records after duplicates removed:- n = 434

Full text article assessed for eli gibility :- n = 36

Abstract/papers excluded because of:

• No empirical data (49)

• Wrong design/sample (158)

• Wrong theme (21 1)• Unknown (8)

Studies in cluded in the analysi s:- n = 8

K.-A. Næss et al. / Research in Developmental Disabilities 33 (2012) 737–747742

Study nam e

Bayli s (2 005))

Roch & Jarrol d (200 8)

-2.00 -1.00 0.00 1.00 2.00

Favour s TD Fav ours DS

Study nam e

Bayli s (2 005)Hulme et al. (submitted )Kay-Raini ng Bir d et al . (2008 )Roch & Jarrol d (200 8)

-2.00 -1.00 0.00 1.00 2.00

Favour s TD Fav ours DS

Fig. 2. Overall mean effect size for vocabulary (displayed by ^) and effect size with confidence interval for each study comparing children with Down

syndrome and reading matched controls.

Study nam e

Coss u et al (1993 )Gombe rt (2 002)Baylis (2005)Verucc i et al (2006 )Hulm e et al. (submit ted )

Bird et al. (2008 )Roch & Jarrold (2 008 )Snowling et al. (2002)

-2.0 0 -1.00 0.00 1.0 0 2.0 0

Fav ours TD Favour s DS

Study nam e

Coss u et al. Gombe rt (2 002)Baylis (2005)Verucci .Hulme et al. )Kay-RainingRoch & Jarrold (2 008 )Snowling et al. (2002)

-2.0 0 -1.00 0.00 1.0 0 2.0 0

Fav ours TD Favour s DS

Fig. 3. Overall mean effect size for phonological awareness (displayed by ^) and effect size with confidence interval for each study comparing children with

Down syndrome and reading matched controls.

K.-A. Næss et al. / Research in Developmental Disabilities 33 (2012) 737–747 743

The Forest plots for the overall mean effect size on vocabulary and phonological awareness are shown in Figs. 2 and 3,respectively. There are significant differences in both vocabulary (g = �0.82, CI [�1.52, �0.12], p = .02) and phonologicalawareness (g = �1.56, CI [�2.04, �1.07], p < .001) between the children with Down syndrome and the typically developingchildren matched for word recognition level.

It should be noted from the Forest plot in Fig. 2 that there is one study that stands out from the other three, namely Rochand Jarrolds’ study (2008). This study used the British Picture Vocabulary Scale (BPVS), as had the other studies, but theirsample had a much higher chronological age (mean 18;11, SD 67 months). This resulted in a positive effect size compared tothe reading matched controls, which actually means that the children with Down syndrome scored higher than the muchyounger typically developing controls.

The results from a moderator analysis showed that differences in vocabulary between the two groups explain 94% of thevariation in nonword decoding, but still only approached level of significance (b = .97, p = .05, k = 4). Differences inphonological awareness between the two groups explain 7% of the variation in nonword decoding between studies, but is nota significant predictor (b = �.26, p = .51).

It should be noted from the Forest plot in Fig. 3 that there is also one study here that stands out from the others. This isCossu et al.’s study (1993), and the results suggest that the previously described criticisms of the measurement in this studymay be correct. However, the pattern is not changed reliably if this study is excluded from the moderator analysis, but itshould be noted that R2 increases to 16%.

When the different phonological awareness measures were used separately in the analysis, they explain more of thevariance than when they were used as a composite. Differences in phoneme deletion between the two groups explain 13.5%of the variation in nonword decoding between studies, but this is not a significant predictor (b = �.37, p = .49), and neither isrhyme detection a significant predictor, although it explain 10.3% of the variation in nonword decoding between the studies(b = .32, p = .50).

Differences in chronological age between children with Down syndrome and the typically developing children explaineda higher proportion of the variance in nonword decoding than differences in phonological awareness. Age explained 43% ofthe variation in nonword reading between studies, but was not a significant predictor (b = �.66, p = .11).

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Due to the small number of studies, these moderator analyses have low power, and this has to be borne in mind wheninterpreting the results of the moderator analyses conducted.

4. Discussion

The purpose of this meta-analytic review was to examine the profile of reading skills in children with Down syndrome bycomparing nonword decoding skills in this group with those seen in typically developing children with the same level ofword recognition ability. A secondary aim was to examine the role of vocabulary and phonological awareness as predictors ofvariance in learning to read. There were no significant differences in nonword decoding skills between the two groups(g = �0.63). However, there was a significant difference between groups in both vocabulary (g = �0.82) and phonologicalawareness (g = �1.56), favouring the typically developing children. Vocabulary (b = .97, p = .05), but not phonologicalawareness (b = �.26, p = .51), was a significant predictor of the variance in nonword decoding. It should be noted thatvariance in vocabulary explained a high proportion of the variance in nonword reading skills, while variance in phonologicalawareness explained a rather modest proportion of the variance. Nevertheless, there is a limited number of studiesexamining decoding skills in children with Down syndrome, and the heterogeneity between studies has to be taken intoaccount when interpreting the results. Given the hypothesis about sight word reading (Byrne et al., 2002; Kay-Raining Bird etal., 2000) and the poor levels of vocabulary and phonological awareness found in children with Down syndrome, we wouldexpect them to have difficulty with nonword decoding. In contrast to this expectation, the results may be seen as support forthe suggestion that decoding skills are a relatively strength in children with Down syndrome. Variables such as vocabularyand phonological awareness are reliably weaker than in the typically developing controls, whereas we found nonworddecoding to be at a slightly, but not significantly, lower level in the DS group. To have decoding skills in line with typicallydeveloping controls may therefore be seen as a relative strength compared to other aspects of the profile of children withDown syndrome. However, the effect size for the nonword decoding was moderate (g = �0.63), and in addition it is importantto note that the control groups of typically developing children are all very young (mean ages between 5.0 and 7.3), whichmeans that they are at a very early stage in the development of nonword decoding skills.

Based on the fact that children with Down syndrome were matched for word recognition level to typically developingchildren in the studies reviewed here, we might assume that phonological awareness would also be at the same level in thetwo groups. However, this is not the case. There were significant differences between the two groups in phonologicalawareness. Further, the fact that phonological awareness did not explain a significant proportion of the variation in nonworddecoding skills in children with Down syndrome is surprising, especially in relation to the conclusions of the qualitativereview by Lemons and Fuchs (2010), which suggested that there is a reliable relationship between phonological awarenessand reading in children with Down syndrome. However, both Snowling et al. (2002) and Cupples and Iacano (2000) reporteda reduced magnitude of relationship between phonological awareness and decoding when controlling for age and cognitiveability. The present meta-analysis is therefore consistent with some previous findings in suggesting that phonologicalawareness is less closely related to decoding skills in children with Down syndrome than that traditionally seen in typicallydeveloping children.

The high proportion of variance in nonword reading skills attributable to vocabulary supports suggestions that languageskills including vocabulary may be important for the development of decoding skills in children with Down syndrome(Hulme et al., under review). Nation and Snowling (2004) provided support for the prediction that vocabulary level is relatedto exception word recognition, but there is no straightforward way to explain how vocabulary knowledge relates to nonworddecoding. The hypothesis by Metsala and Walley (1998) suggests that vocabulary predicts phonological awareness. If this isthe case, phonological awareness will gradually develop as vocabulary increases. This means that larger vocabulary size willbe associated with better phonological awareness. Nevertheless, as already mentioned, most children with Down syndromehave phonological awareness skills at a relatively low level compared to typically developing controls. They may, however,have reached a level of phonological awareness that is satisfactory for decoding nonwords, but which is too low to give crediton the phonological awareness tests they have undertaken. This leads us to ask what level of phonological ability is necessaryto decode nonwords. Based on the present results, we suggest that nonword decoding depends primarily on knowledge ofgrapheme–phoneme correspondences and phonological memory rather than the ability traditionally assessed by tests ofphonological awareness. We do not know what level of phonological awareness is necessary for decoding nonwords or howthis relationship changes with developmental level. This means that the weakness in phonological awareness seen inchildren with Down syndrome may be a critical factor for proficient decoding skills in children with Down syndrome.

Another explanation of the high impact of vocabulary on nonword decoding skills can be due to vocabulary as a factor ofthe child’s mental ability. In this view, a certain level of vocabulary is necessary to understand the tasks of decodingmeaningless nonwords, and children with low vocabulary levels will possibly try to decode nonwords as real words theyhave heard before.

4.1. Implications for practise and future research

If children with Down syndrome have nonword decoding skills in line with the performance of typically developingchildren matched on word recognition level, it should influence our decisions about what reading strategies to teach forchildren with this diagnosis. The present meta-analysis suggests that children with Down syndrome can use the

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alphabetic principle in order to decode words to a certain extent. This suggests that children with Down syndrome maybenefit from being taught phonetic decoding strategies. If children with Down syndrome with a low level ofphonological awareness learn to decode nonwords, teaching methods should possibly focus on helping these children tounderstand the alphabetic principle and to decode short words that do not place excessive demands on their limitedverbal short-term memory skills.

Traditionally, phonetic decoding interventions for children with Down syndrome as for typically developing childrenhave focused on phonological awareness skills. (For narrative reviews of reading intervention in children with Downsyndrome, see Burgoyne, 2009; Snowling et al., 2008). However, if there is a limit to what level of phonologicalawareness children with Down syndrome might reach, one should look for alternative ways of developing decodingskills.

Due to the close relationship between vocabulary knowledge and nonword decoding, it seems reasonable to prioritiselanguage and especially vocabulary as an important factor in reading intervention programmes for children with Downsyndrome. To meet the cognitive level often seen in individuals with this diagnosis, direct teaching of words, and learning byexperiences rather than indirect learning of different word learning strategies, may be beneficial. Also, repeated exposure tothe same activity and words may increase the possibility to automatise new vocabulary. Both vocabulary training focusingon words that will be useful in everyday life, and vocabulary focusing on the content of the words/text and the readingprocess such as line, direction, and names of the letters, may be included in the programme in order to increase vocabularysize and help students learn what reading is about.

If vocabulary training also gives support to the development of phonological awareness, vocabulary training should beincluded in a kind of combined language and decoding programme. Such a combined programme is also positive, in thatchildren with developmental disabilities in general often struggle with transforming a skill from one situation to another. Tolearn to read in a natural setting, combining the skills included in the decoding process, may therefore be beneficial.However, there is limited evidence for either reading development in children with Down syndrome or for the effectivenessof different teaching approaches for these children, which means that further research into the effectiveness of decodingintervention or compensatory strategies focusing, e.g., on vocabulary training, is badly needed.

5. Summary and conclusions

The results of the present meta-analysis are generally in line with earlier research suggesting that decoding skills are arelative strength in children with Down syndrome, and adding that the limited number of studies in this area means thatmore research is needed. The role of vocabulary knowledge in learning to decode words has also been highlighted. Childrenwith Down syndrome perform in line with typically developing children at the same word recognition level on measures ofnonword decoding, but show deficits on measures of the two underlying skills: vocabulary and phonological awareness.Differences in vocabulary explain substantial variance in nonword decoding skills, while phonological awareness did nothave as much impact as traditionally seen in studies of typically developing children. We suggest that early vocabularyinterventions for children with Down syndrome may be beneficial to their development of decoding skills, but suchprogrammes need to be rigorously evaluated in future research studies.

Appendix A

See Table A1.

Table A1

Characteristics of the studies comparing children with Down syndrome and typically developing children at the same reading level.

Study author, year N Mean CA Measure Effect size, g 95% CI

DS (TD) DS (TD)

Kay-Raining Bird et al. (2008) 20 (17) 8;6–19;10

(4;9–10;9)

Nonword reading �0.68 �1.33 to �0.02

Word reading �0.23 �0.86 to 0.41

Receptive vocabulary 1.00 �1.61 to �0.27

PA �1.13 �1.82 to �0.45

Cossu et al. (1993) 10 (10) 11;4 (7;3) Nonword reading 0.60 �0.26 to 1.46

Word reading �0.36 �1.2 to 0.48

PA �4.33 �6.58 to �3.13

Gombert (2002) 11 (11) 13;9 (7;1) Nonword reading �0.27 �1.07 to 0.54

Word reading �0.02 �0.83 to 0.78

PA �2.23 �3.27 to �1.19

Hulme et al. (under review) 49 (61) – Nonword reading �1.64 �2.08 to �1.21

Word reading �0.12 �0.49 to 0.26

Receptive vocabulary �1.02 �1.42 to �0.62

PA �1.16 �1.56 to �0.76

Table A1 (Continued )

Study author, year N Mean CA Measure Effect size, g 95% CI

DS (TD) DS (TD)

Baylis (2005) 18 (18) 11;6 (6;8) Nonword reading �2.11 �2.91 to �1.3

Word reading 0.05 �0.59 to 0.69

Receptive vocabulary �1.59 �2.32 to �0.85

PA �2.16 �2.98 to �1.35

Roch and Jarrold (2008) 12 (14) 18;11 (6;10) Nonword reading 1.06 0.26–1.86

Word reading �0.53 �1.29 to 0.23

Receptive vocabulary 0.40 �0.35 to 1.16

PA �0.97 �1.76 to �0.17

Snowling et al. (2002) 29 (31) 13;2 (5;3) Nonword reading �0.32 �0.55 to 0.44

Word reading 0.05 �0.42 to 0.57

PA �0.87 �1.39 to �0.35

Nonword reading �0.07 �0.58 to 0.73

Verucci et al. (2006) 17 (17) 16;5 (7;0) Nonword reading �1.57 �0.82 to 0.18

Word reading �0.05 �0.55 to 0.44

PA �1.44 �1.39 to �0.35

K.-A. Næss et al. / Research in Developmental Disabilities 33 (2012) 737–747746

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