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Effectiveness of Treatment Approaches for Children and Adolescents with Reading Disabilities: A Meta-Analysis of Randomized Controlled Trials Katharina Galuschka1, Elena Ise2, Kathrin Krick1, Gerd Schulte-Körne1*
1 Department of Child and Adolescent Psychiatry and Psychotherapy, University of Munich, Munich, Germany, 2 Department of Child and Adolescent Psychiatry and
Psychotherapy, University of Cologne, Cologne, Germany
Abstract
Children and adolescents with reading disabilities experience a significant impairment in the acquisition of reading and spelling skills. Given the emotional and academic consequences for children with persistent reading disorders, evidence- based interventions are critically needed. The present meta-analysis extracts the results of all available randomized controlled trials. The aims were to determine the effectiveness of different treatment approaches and the impact of various factors on the efficacy of interventions. The literature search for published randomized-controlled trials comprised an electronic search in the databases ERIC, PsycINFO, PubMed, and Cochrane, and an examination of bibliographical references. To check for unpublished trials, we searched the websites clinicaltrials.com and ProQuest, and contacted experts in the field. Twenty-two randomized controlled trials with a total of 49 comparisons of experimental and control groups could be included. The comparisons evaluated five reading fluency trainings, three phonemic awareness instructions, three reading comprehension trainings, 29 phonics instructions, three auditory trainings, two medical treatments, and four interventions with coloured overlays or lenses. One trial evaluated the effectiveness of sunflower therapy and another investigated the effectiveness of motor exercises. The results revealed that phonics instruction is not only the most frequently investigated treatment approach, but also the only approach whose efficacy on reading and spelling performance in children and adolescents with reading disabilities is statistically confirmed. The mean effect sizes of the remaining treatment approaches did not reach statistical significance. The present meta-analysis demonstrates that severe reading and spelling difficulties can be ameliorated with appropriate treatment. In order to be better able to provide evidence-based interventions to children and adolescent with reading disabilities, research should intensify the application of blinded randomized controlled trials.
Citation: Galuschka K, Ise E, Krick K, Schulte-Körne G (2014) Effectiveness of Treatment Approaches for Children and Adolescents with Reading Disabilities: A Meta-Analysis of Randomized Controlled Trials. PLoS ONE 9(2): e89900. doi:10.1371/journal.pone.0089900
Editor: Karen Lidzba, University Children’s Hospital Tuebingen, Germany
Received July 6, 2013; Accepted January 29, 2014; Published February 26, 2014
Copyright: � 2014 Galuschka et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This research was supported by grants from the Bundesverband für Legasthenie und Dyskalkulie e.V. (BVL) and Deutsche Gesellschaft für Kinder- und Jugendpsychiatrie, Psychosomatik und Psychotherapie (DGKJP). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: [email protected]
Introduction
Children, adolescents, and adults with reading disability
(dyslexia) experience a significant impairment in the acquisition
of reading accuracy, reading fluency, reading comprehension, and
spelling skills, which cannot be accounted for by low IQ, visual
acuity problems, neurological damage, or poor educational
opportunities [1]. Reading disability has genetic basis [2] and
the underlying neurobiological and cognitive causes are largely
debated. Impairments in auditory speech perception and process-
ing, as well as visual attention and perception deficits are
considered as the main causes of reading and spelling difficulties
in dyslexia [3–5]. Reading and spelling deficits influence an
individual’s performance in most academic domains [1]. In
addition, there is strong evidence of a link between reading
disabilities and externalizing disorders, generalized anxiety, and
school-related anxiety [6,7].
The evidence-based development and the evaluation of
interventions for children and adolescents with reading disabilities
are, therefore, of particularly profound importance. A large
number of interventions and therapies, derived from various
treatment approaches, have been constructed and evaluated.
Several systematic reviews have already summarized the findings
of studies that evaluated the effectiveness of reading and spelling
interventions. One of the most influential reviews of the research
literature was conducted by the National Reading Panel (NRP) in
the year 2000 [8]. The review displays important results about the
effectiveness of different types of reading instruction. Its main
finding was that systematic instruction in learning letter sound
relations and in blending sounds to form words is most effective for
improving reading and spelling skills in disabled readers [8].
Despite the importance of this finding, 13 years after its
publication, the NRP review needs to be updated in order to
integrate recent empirical findings.
However, most current systematic reviews are focused on
the effectiveness of one specific treatment approach [9–11]. Other
reviews address preventive methods for children at-risk for
reading disability [12,13]. Since there is no widespread use of
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randomized-controlled trials (RCTs) in this research domain,
current systematic reviews and meta-analyses often included not
only RCTs, but also low-quality primary research (e.g., non-
randomized research designs) [14–16]. To our best knowledge, no
systematic review has been published to date that includes all
available RCTs, without focusing on a specific treatment
approach, and that integrates the results quantitatively with
statistical methods.
The present meta-analysis has two advantages over previously
published work. First, due to the inclusion of exclusively RCTs, the
observed effect sizes can most likely be attributed to the
intervention. Second, because all available RCTs are integrated,
it is possible to compare the effectiveness of different treatment
approaches.
The goal of this meta-analysis is twofold. The first aim is to
determine the efficacy of different treatment approaches on
reading and spelling performance of reading disabled children and
adolescents. The second aim is to explore the impact of various
factors on the efficacy of these treatment approaches.
Methods
Literature Search An extensive literature search was conducted. We searched for
intervention studies that were published until June 2013 in the
databases ERIC, PsycINFO, PubMed, and Cochrane with the
following search terms: ‘‘dyslexia’’ or ‘‘developmental reading
disorder’’ or ‘‘developmental dyslexia’’ or ‘‘developmental reading
disability’’ or ‘‘reading disorder’’ or ‘‘word blindness’’ or ‘‘spelling
disorder’’ or ‘‘developmental spelling disorder’’ or ‘‘specific
spelling disorder’’ combined with ‘‘intervention’’ or ‘‘treatment’’
or ‘‘therapy’’ or ‘‘therapeutics’’ or ‘‘training’’ or ‘‘remediation’’.
We also examined bibliographical references of systematic
reviews and primary studies. To check for unpublished RCTs, we
searched the websites clinicaltrials.com and ProQuest. In addition,
we contacted experts by sending an e-mail to each member of the
mailing list of the Society for the Scientific Studies of Reading
(SSSR).
Study Selection Criteria To be considered for this review, studies must have met the
following criteria: (a) the aim of the study was to examine the
efficacy of an intervention or remediation programme for children
and adolescents with reading disabilities; (b) the manuscript was
written in English; (c) the study design included an untrained
control group or a placebo training group; (d) group allocation was
randomized, including parallel group randomization, group
cluster randomization (quasi-randomized controlled trials were
not selected); (e) study subjects were children, adolescents or adults
(no studies with adults could be included) whose reading
performance was below the 25 th
percentile or at least one
standard deviation (SD), one year, or one grade below the
expected level; (f) the study included subjects with intelligence in
the normal range (IQ$85, or described as having normal
intelligence by the study author); (g) poor reading occured in
mother tongue; (h) one or more reading or spelling tests were
administered before and after treatment; and (i) pre- and post-test
results of the reading or spelling tests were reported with sufficient
detail to allow the calculation of an effect size or could be
requested from the authors. Figure 1 summarizes the process of
selecting studies for the meta-analysis.
Coding of the RCTs Coding was done independently by the first author and an
associate using a structured coding sheet. First, data necessary for
effect size calculation (mostly means and standard deviations of
pre- and post-tests) was extracted. Next, methodological charac-
teristics, intervention characteristics, and sample characteristics
were coded.
The methodological characteristics included: (a) the dependent
variable (reading speed, reading comprehension, reading accura-
cy, pseudoword reading accuracy, pseudoword reading speed,
nonword reading accuracy, nonword reading speed, or spelling);
(b) the sample size; and (c) the administered reading test and
spelling test. The intervention characteristics included: (a) treat-
ment approach; (b) spelling/writing activities included (yes or no);
(c) duration of the intervention in weeks; (d) total amount of
intervention in hours; (e) setting (group, individual, or computer);
and (f) conductor (professional or nonprofessional).
Treatment approaches were classified into distinct categories
based on the description of the intervention in the report. The
categories closely match the topic areas of the NRP review [8].
The category phonemic awareness instruction includes interventions
that foster the ability to recognize and manipulate phonemes in
words. This implies tasks for recognizing phonemes in words,
blending phonemes to words, segmenting a word into its
phonemes, eliminating a phoneme from a word, or adding a
phoneme to a word. All tasks are presented and performed orally.
The category phonics instruction includes interventions that system-
atically teach letter-sound-correspondences and decoding strate-
gies that involve blending or segmenting individual letters or
phonemes or dividing a spoken or written word into syllables or
onset and rimes. These interventions comprise reading and writing
activities. The category reading fluency training includes interventions
that contain repeated oral word reading practice or guided
repeated word reading. These interventions aim to improve word
recognition skills. The category reading comprehension training includes
interventions that comprise tasks in which participants learn to
extract textual information, summarize it, and relate it to existing
knowledge. The category auditory training includes interventions in
which subjects are confronted with non-linguistic auditory stimuli
and are trained to identify and distinguish these stimuli. The
category medical treatment includes interventions in which partici-
pants receive drugs to enhance their reading and spelling
performance. The category coloured overlays includes interventions
in which study subjects read with coloured filters or coloured
overlays.
Finally, sample characteristics were coded. These included (a)
age (mean and range) and (b) severity of reading impairment. The
severity of reading impairment was identified by the inclusion
criteria used in the trials and consists of three categories. The
category severe reading disability includes studies in which partici-
pants’ reading performance was at least 2 SD below the expected
value, below the 2.5 th
percentile, at least two years below grade
level, or showed a discrepancy between chronological age and
reading age of at least two years. The category moderate reading
disability includes studies in which participants’ reading perfor-
mance was at least 1 SD below the expected value, at least one
year below grade level, below the 16 th
percentile, or showed a
discrepancy between chronological age and reading age of at least
one year. The category mild reading disability includes studies in
which participants’ reading performance was below the 25 th
percentile.
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Data Extraction and Effect Size Calculation To evaluate the efficacy of an intervention, the effect size
Hedges g was calculated by dividing the difference between the
performance scores of the control group (CG) and the experi-
mental group (EG) at post-test by their pooled standard deviation,
and multiplying the result by a correction factor [17,18].
Formula 1 - Hedges g
g~ MEG{MCGffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
n EG
{1ð Þs2EG z nEG {1ð Þs2CG n
EG zn
CG {2
r 1{ 3 4df {1
M = mean; EG = experimental group; CG = control group;
n = number of study subjects; s = standard deviation; df = degrees
of freedom.
If studies included more than one intervention group, but only
one control group, every comparison of an intervention group
with the control group was treated separately as an individual
study. As a consequence, the control group was used to compute
several effect sizes which are not independent from each other. An
overweighting of the effect sizes was counteracted by dividing the
sample size of the control group by the number of intervention
groups. Similarly, if several control groups, but only one
intervention group, were included, each comparison of a control
group with the intervention group was treated as an individual
study and the sample size of the intervention group was divided by
the number of control groups.
To reduce the risk of under- or overestimating effect sizes, some
effect sizes were corrected for pre-test differences. If the difference
between the pre-test scores of the experimental and the control
group displayed an effect size equal or greater than 0.20 (g$0.20),
the post-test score of the experimental group was corrected by
adding or subtracting the difference between the pre-test scores.
The effect size was then calculated on the basis of the corrected
post-test score and the (uncorrected) pooled standard deviation.
This was done because the formula described above does not take
into consideration the pre-test differences, which leads to an over-
or underestimation of the true magnitude of the effect if there are
significant differences between the groups before the start of the
intervention.
A maximum of two effect sizes were calculated for each
comparison of an experimental group with a control group, one
for reading performance and one for spelling performance. The
following measures of reading performance were considered
adequate for effect size calculation: reading accuracy, reading
Figure 1. Flow chart of study selection process. doi:10.1371/journal.pone.0089900.g001
Effectiveness of Interventions for Poor Readers
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speed, reading comprehension, nonword reading speed, nonword
reading accuracy, pseudoword reading speed or pseudoword
reading accuracy. To determine spelling performance, tests
measuring spelling accuracy were considered adequate.
Some studies used multiple reading and spelling tests to
determine treatment efficacy, including standardized measures
and non-standardized measures of learning transfer, as well as
non-standardized measures whose tasks closely matched the
training content. Effect sizes were calculated based on standard-
ized measures, which are generally considered to be measures of
learning transfer, if these were available. If standardized measures
were not available, non-standardized measures of learning transfer
were used for effect size calculation (n = 3 studies). Self-constructed
measures that matched the training content were not used for
effect size calculation, because these measures may not generalize
to material not specifically taught. Thus, all effect sizes are based
on measures of learning transfer. If a study reported results for
several comparable tests (e.g., several standardized tests measuring
different aspects of reading such as reading speed and compre-
hension), an average effect size was calculated from the effect sizes
for individual tests, separately for reading and spelling perfor-
mance.
Non-standardized dependent measures are suspected to over-
estimate the true magnitude of an effect [14,19]. Although all
effect sizes are based on measures of learning transfer, it cannot be
ruled out completely that the inclusion of studies without
standardized measures introduced an artifact. For this reason,
the main analyses were run with and without studies that used
non-standardized measures. First, the analyses were conducted
with all studies that met the inclusion criteria outlined above (i.e.
studies with standardized or non-standardized measures; n = 22
studies; see Table 1). Second, the analyses were run with those
studies that included standardized measures (n = 19 studies).
For studies that did not report means and standard deviations,
effect sizes were calculated on the basis of other measures, for
example t-test or F-test values. If a study did not report sufficient
data, more information was requested from the corresponding
author. If this request failed, co-authors were contacted.
Quality Assessment The methodological quality of the included studies was assessed
independently by the first author and an associate with the
checklist for randomized controlled trials by the Scottish
Intercollegiate Guidelines Network. To assess selection bias, it
was determined if an adequate concealment method was used.
Centralised allocation, computerised allocation systems, and the
use of opaque envelopes were regarded as adequate methods of
concealment. To assess performance/detection bias, it was
determined if the study was blinded. Blinding of the participants
and therapists is difficult to ensure in cognitive treatment trials.
Therefore, it was only appraised if the assessment of the outcome
measures was conducted by a blinded person. To assess reporting
bias, it was determined if the data was adequately reported.
Statistical Analysis All analyses were performed using Biostat software ‘‘Compre-
hensive Meta Analysis Version 2.2.064’’ [20]. Because of
substantial differences between the treatment approaches that
were evaluated in the included studies, there is no reason to
assume that all studies share an identical true effect size.
Consequently, a random effects model was used for the meta-
analysis.
Results
Of the randomized-controlled trials that were identified by the
literature search, only 22 met all inclusion criteria and could be
included in the meta-analysis. Interrater-agreement for article
inclusion or exclusion exceeded k = 0.786. All discrepancies were resolved by discussion. Coding reliabilities (percentage of inter-
rater-agreement) for study characteristics and data extraction
averaged 87%. Again, all discrepancies were disputed and solved.
Specifications regarding the methodological quality of the
included trials were often incomplete. A sufficient description of
the allocation concealment was missing in each of the 22 trials.
Sixteen trials did not specify if the dependent variables were
assessed by a blinded person [21–36]. Two trials [37,38] stated
explicitly that the outcome measures were assessed by a person
that was aware of the study subjects’ affiliation. Four studies [39–
42] performed a blind assessment of treatment outcomes. It can
therefore be concluded that most studies are at risk of having a
bias. Data was considered as adequately reported in all of the
included trials. One trial had to be excluded from the analysis due
to lack of information regarding outcome data. Attempts to
contact the authors failed.
Table S1 presents an overview of the trials that are included in
the meta-analysis. Thirteen of the 22 trials included more than one
intervention group, and two trials included more than one control
group. Therefore, the meta-analysis was computed with a total of
49 comparisons of an experimental and a control group. These
comparisons comprised 1138 participants in the experimental
groups and 764 participants in the control groups. Effect size data
for each subgroup within a study is presented separately for
reading and spelling performance in Figure 2 and Figure 3.
Table 1. Efficacy of treatment approaches on reading performance.
95% CI Heterogeneity Significance
Variable Value N g’ Lower Upper Q df p I2 Q df p
Treatment approach Phonemic awareness instruction 3 0.279 20.244 0.802 3.663 2 0.160 45% 3.164 6 0.788
Phonics instruction 29 0.322 0.177 0.467 26.810 28 0.529 0%
Reading fluency training 5 0.301 20.105 0.707 1.389 4 0.845 0%
Reading comprehension training 3 0.177 20.181 0.535 0.525 2 0.769 0%
Auditory training 3 0.387 20.065 0.838 2.053 2 0.358 3%
Medical treatment 2 0.125 20.072 0.322 1.331 1 0.249 25%
Coloured overlays 4 0.316 20.012 0.644 0.885 3 0.829 0%
doi:10.1371/journal.pone.0089900.t001
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The comparisons were distributed across the treatment
approaches as follows: five reading fluency trainings, three
phonemic awareness instructions, three reading comprehension
trainings, 29 phonics instructions, three auditory trainings, two
medical treatments, and four coloured overlays or lenses. One trial
evaluated the effectiveness of sunflower therapy and another
investigated the effectiveness of specific motor sequences. These
two interventions could not be allocated to a category because they
pursue an entirely different treatment approach. Results of the
meta-analysis are reported separately for reading and spelling
performance.
Reading Performance All included studies reported the results of reading measures,
which made it possible to estimate each intervention’s efficacy
regarding reading performance. Phonics instruction was investi-
gated most often. This approach is the only one whose
effectiveness on reading performance was statistically confirmed.
The mean effect size for phonics instruction was g’ = 0.322 (95%
CI [0.177, 0.467]; I 2
= 0). This suggests a small but statistically
significant effect of phonics instructions on reading performance.
The I 2
statistic describes the proportion of observed dispersion that
reflects real differences rather than differences that occur by
chance. As can be seen in Table 1, the mean effect sizes of the
remaining treatment approaches did not reach statistical signifi-
cance. Subgroup analysis revealed no statistically significant
difference between treatment approaches (p = .788).
In addition, subgroup analyses were conducted to explore the
influence of other variables (intervention and sample characteris-
tics) on reading improvement. Results are displayed in Table 2.
Studies that did not include or did not specify a certain variable
were excluded from the subgroup analysis in question. In addition,
it was not possible to define subgroups of age or grade level
because children’s age and grade level showed considerable
overlap between studies. Therefore, it was not possible to perform
subgroup analyses with these variables.
The analyses revealed that intervention studies with mild
reading disabled children and adolescents report a slightly higher
mean effect size (g’ = 0.449; 95% CI [0.239, 0.659]; I 2
= 0%)
compared with studies that included moderately disabled
(g’ = 0.228; 95% CI [0.113, 0.342]; I 2
= 31%) or severe reading
Figure 2. Treatment efficacy on reading performance. Funnel plot displays treatment efficacy on reading performance for each comparison of an experimental group with a control group. ADD = Adding phonemes; CG = Control group; DI = Direct instruction; DS = Decoding skills; EXC = Exceptional; LPA = Sound-symbol correspondence and phonemic awareness; MA = Morphological awareness; OWLS = Oral and written language skills; PADS = Phonemic awareness and decoding skills; PAT = Phonological awareness training; PG = Placebo-control group; PHAB = Phonological analysis and blending; REG = Regular; SMS = Specific motor sequence; SP = Speech perception; SRT = Strategy reciprocal teaching; TCS = Text content and structure; TOD = Temporal order detection; WIST = Word identification strategy training; WAT = Word analogy training; WW = Write a word. doi:10.1371/journal.pone.0089900.g002
Effectiveness of Interventions for Poor Readers
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disabled (g’ = 0.305; 95% CI [0.033, 0.576]; I 2
= 0%) study
subjects. However, this difference did not reach statistical
significance (p = .188).
Studies were allocated into three distinct subgroups depending
on the amount of intervention that was provided. No significant
difference (p = .250) was found between the mean effect size of
interventions that lasted up to 14 hours (g’ = 0.351; 95% CI [0.181,
0.520]; I 2
= 0%), interventions that lasted between 15 hours and
34 hours (g’ = 0.113; 95% CI [20.148, 0.374]; I 2
= 0%), and
interventions with more than 35 hours (g’ = 0.371; 95% CI [0.172,
0.570]; I 2
= 0%).
To compare the effects of interventions with short- and long-
term duration, the studies were divided into two subgroups: (a) up
to 12 weeks; and (b) more than 12 weeks. The cut-off value of 12
weeks was chosen because it results in two subgroups of equal size
making a statistical comparison between the two groups more
appropriate. Interventions with a maximum duration of 12 weeks
showed a small mean effect size of g’ = 0.261 (95% CI [0.155,
0.368]; I 2
= 0%). Interventions that lasted more than 12 weeks
tended to show higher effect sizes (g’ = 0.353; [0.151, 0.554];
I 2
= 12%). Again, this difference did not reach statistical signifi-
cance (p = .432).
Figure 3. Treatment efficacy on spelling performance. Funnel plot displays treatment efficacy on spelling performance for each comparison of an experimental group with a control group. ADD = Adding phonemes; CG = Control group; DI = Direct instruction; DS = Decoding skills; EXC = Exceptional; MA = Morphological awareness; OWLS = Oral and written language skills; PAT = Phonological awareness training; PG = Placebo- control group; PHAB = Phonological analysis and blending; REG = Regular; SMS = Specific motor sequence; WIST = Word identification strategy training; WAT = Word analogy training; WW = Write a word. doi:10.1371/journal.pone.0089900.g003
Table 2. Subgroup analyses to explore the influence of variables on reading performance.
95% CI Heterogeneity Significance
Variable Value N g’ Lower Upper Q df p I2 Q df p
Severity Mild reading disability 20 0.449 0.239 0.659 2.893 19 1.000 0% 3.339 2 0.188
Moderate reading disability 23 0.228 0.113 0.342 32.037 22 0.077 31%
Severe reading disability 9 0.305 0.033 0.576 4.508 8 0.000 0%
Amount Up to 14 hours 17 0.351 0.181 0.520 16.023 16 0.450 0% 2.774 2 0.250
Between 15 hours and 34 hours 12 0.113 20.148 0.374 10.650 11 0.473 0%
35 hours and more 15 0.371 0.172 0.570 5.747 14 0.972 0%
Duration Up to 12 weeks 35 0.261 0.155 0.368 23.927 34 0.901 0% 0,618 1 0.432
More than 12 weeks 17 0.353 0.151 0.554 18.231 16 0.311 12%
Setting Computer with teacher 9 0.364 0.085 0.643 2.766 8 0.948 0% 1.818 2 0.403
Single subject 11 0.205 0.003 0.407 23.503 10 0.009 57%
Group 22 0.379 0.211 0.549 6.173; 21 0.999 0%
Conductor Study autor 5 0.806 0.397 1.215 6.446 4 0.168 38% 6.543 3 0.088
Student 3 0.400 20.109 0.909 0.144 2 0.931 0%
Teacher 13 0.247 0.046 0.449 6.046 12 0.914 0%
Special education therapist 20 0.256 0.090 0.422 13.622 19 0.805 0%
Spelling/writing Included 6 0.152 20.157 0.451 7.332 5 0.197 32% 1.137 1 0.286
Not included 34 0.331 0.195 0.467 24.473 33 0.858 0%
doi:10.1371/journal.pone.0089900.t002
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To detect the impact of the setting on the success of an
intervention three subgroups could be differentiated: (a) computer
with teacher; (b) individual intervention; and (c) group interven-
tion. The mean effect sizes of these subgroups did not differ
significantly from each other (p = .403). The studies in the
computer with teacher subgroup reached a mean effect size of
g’ = 0.364 (95% CI [0.085, 0.643]; I 2
= 0%), which was compa-
rable to the mean effect size of group interventions (g’ = 0.379;
95% CI [0.211, 0.549]; I 2
= 0%). Single subject interventions
showed a small but significant mean effect size of g’ = 0.205 (95%
CI [0.003, 0.407]; I 2
= 57%).
Interventions that were conducted by the study author showed a
high mean effect size (g’ = 0.806; 95% CI [0.397, 1.215];
I 2
= 38%), whereas interventions that were conducted by teachers
(g’ = 0.247; 95% CI [0.046, 0.449]; I 2
= 0%) or special education
therapists (g’ = 0.256; 95% CI [0.090, 0.422]; I 2
= 0%) led to
negligible mean effect sizes. Interventions that were conducted by
students reached a small mean effect size (g’ = 0.400; [20.109,
0.909]; I 2
= 0%). Although a trend could be identified, there was
no significant difference between these subgroups (p = .088).
In addition, subgroup analysis showed that the mean effect size
of studies that did not include spelling/writing activities is
moderate and significantly greater than zero (g’ = 0.331; 95% CI
[0.195, 0.467]; I 2
= 0%). Interventions that included spelling/
writing exercises showed a small effect on reading improvement
that did not reach statistical significance (g’ = 0.152; 95% CI
[20.157, 0.451]; I 2
= 32%). This difference did not reach
statistical significance (p = .286).
Spelling Performance Ten trials (containing 18 comparisons) conducted spelling tests
before and after treatment. It was, therefore, possible to calculate
18 effect sizes for spelling. Only in case of phonics instruction was
it possible to compute a mean effect size. The other treatment
approach categories included only one study that assessed spelling
performance. Ten studies evaluated the effect of phonics
instruction on spelling performance. These revealed a small but
statistically significant mean effect size (g’ = 0.336; 95% CI [0.062,
0.610]; I 2
= 22%).
Again, subgroup analyses were conducted to explore the
involvement of other variables (intervention and sample charac-
teristics) on the improvement of spelling performance. Because
only few studies were available, some subgroups comprised less
categories as in the case of reading performance (see Table 3).
Studies with participants considered as mild reading disabled
(g’ = 0.415; 95% CI [0.089, 0.741]; I 2
= 0%) showed a statistically
significant mean effect size on spelling performance, whereas the
effectiveness of studies with moderately disabled study subjects
(g’ = 0.157; 95% CI [20.027, 0.340]; I 2
= 28%) could not be
statistically confirmed. However, the analysis revealed no statis-
tically significant difference between these two categories of
severity (p = .176).
Significant differences (p = .010) were found between the mean
effect sizes of interventions that lasted up to 14 hours (g’ = 0.432;
95% CI [0.114, 0.749]; I 2
= 14%), interventions that lasted
between 15 hours and 34 hours (g’ = 1.140; 95% CI [0.404,
1.875]; I 2
= 0%), and interventions with more than 35 hours
(g’ = 0.059; 95% CI [20.181, 0.300]; I 2
= 0%). In contrast, it was
found that interventions that lasted more than 12 weeks have a
higher mean effect size (g’ = 0.314; [20.015, 0.643]; I 2
= 0%) than
interventions with a maximum duration of 12 weeks (g’ = 0.176;
[0.011, 0.341]; I 2
= 13%). However, this difference failed to reach
statistical significance (p = .462).
Interventions that were conducted by teachers (g’ = 0.099; 95%
CI [20.412, 0.610]; I 2
= 0%) or special education therapists
(g’ = 0.148; 95% CI [20.082, 0.378]; I 2
= 23%) led to negligible
mean effect sizes. Interventions that were conducted by students
reached a large mean effect size (g’ = 0.945; 95% CI [0.417,
1.474]; I 2
= 0%). This difference reached statistical significance
(p = .021).
The mean effect sizes of studies that investigated individually
administered interventions and studies that investigated group
interventions did not differ significantly from each other (p = .476).
Single subject interventions showed a mean effect size of
g’ = 0.488, which was not statistically greater than zero (95% CI
[20.061, 1.038]; I 2
= 48%). Group interventions showed a mean
effect size of g’ = 0.266 (95% CI [0.000, 0.532]; I 2
= 14%).
The mean effect size of studies that did not include spelling/
writing activities (g’ = 0.337; 95% CI [20.038, 0.713]; I 2
= 14%)
did not significant differ (p = .908) from the mean effect size of
interventions that included spelling/writing exercises (g’ = 0.371;
95% CI [20.067, 0.809]; I 2
= 49%).
Additional Analyses In the vast majority of studies (19 out of 22), the effect size
calculation was based on standardized measures. Only three trials
[23,26,30] used non-standardized measures of learning transfer.
These studies had evaluated phonics instructions, reading com-
prehension trainings, and a reading fluency training. Because the
inclusion of studies with non-standardized measures might
introduce an artifact (outlined above), the main analyses were
rerun after these three studies were excluded.
Since only one study remained in the category ‘reading
comprehension training’, it was not possible to calculate a mean
effect size for this treatment approach. In the category ‘reading
fluency training’ the exclusion of studies with non-standardized
measures led to a minor change in the magnitude of the effect
(Reading: g’ = 0.280; 95% CI [20.072, 0.322]); n = 4). Interest-
ingly, the mean effect sizes for phonics instruction are higher if
trials using non-standardized measures are excluded from the
analysis (Reading: g’ = 0.424; 95% CI [0.246, 0.601]; n = 25;
Spelling: g’ = 0.376; 95% CI [0.065, 0.686]); n = 9). These findings
demonstrate that the inclusion of studies with non-standardized
measures in the present meta-analysis did not lead to an
overestimation of the effect sizes and, therefore, does not confound
the results.
Publication Bias A common problem of all disciplines in meta-analytic reviews is
the phenomenon of publication bias [43]. Publication bias occurs
because statistically significant results are more likely to be
published than non-significant results.
Only a small number of included studies assessed spelling
performance. In addition, phonics instruction is the only treatment
approach whose positive effect on reading performance is
statistically confirmed. Therefore, publication bias was explored
exemplarily only for those studies that evaluated phonics
instruction and used reading performance as dependent variable.
A funnel plot was used to explore the presence of publication bias.
The shape of the funnel plot displayed asymmetry with a gap on
the left of the graph. Using Duval and Tweedie’s trim and fill [44]
the extent of publication bias was assessed and an unbiased effect
size was estimated. This procedure trimmed 10 studies into the
plot and led to an estimated unbiased effect size of g’ = 0.198 (95%
CI [0.039, 0.357]) (see Figures 4 and 5, Table 4).
Effectiveness of Interventions for Poor Readers
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Table 3. Subgroup analyses to explore the influence of variables on spelling performance.
95% CI Heterogeneity Significance
Variable Value N g’ Lower Upper Q df p I2 Q df p
Severity Mild reading disability 8 0.415 0.089 0.741 4.965 7 0.664 0% 1.830 1 0.176
Moderate reading disability 8 0.157 20.027 0.340 9.712 7 0.205 28%
Amount Up to 14 hours 4 0.432 0.114 0.749 3.481 3 0.323 14% 9.295 2 0.010
Between 15 hours and 34 hours 3 1.140 0.404 1.875 0.589 2 0.745 0%
35 hours and more 8 0.059 20.181 0.300 2.620 7 0.918 0%
Duration Up to 12 weeks 9 0.176 0.011 0.341 9.209 8 0.325 13% 0.542 1 0.462
More than 12 weeks 9 0.314 20.015 0.643 7.061 8 0.530 0%
Setting Single subject 3 0.488 20.061 1.038 3.817 2 0.148 48% 0.509 1 0.476
Group 11 0.266 0.000 0.532 11.565 10 0.315 14%
Conductor Student 3 0.945 0.417 1.474 0.007 2 0.997 0% 7.734 2 0.021
Teacher 4 0.099 20.412 0.610 0.417 3 0.937 0%
Special education therapist 7 0.148 20.082 0.378 7.793 6 0.254 23%
Spelling/writing Included 5 0.371 20.067– 0.809 7.814 4 0.099 49% 0.013 1 0.908
Not included 8 0.337 20.038 0.713 8.111 7 0.323 14%
doi:10.1371/journal.pone.0089900.t003
Figure 4. Funnel plot of standard error by Hedges g for observed comparisons. Funnel plot displays observed comparisons evaluating the efficacy of phonics instructions on reading performance. doi:10.1371/journal.pone.0089900.g004
Effectiveness of Interventions for Poor Readers
PLOS ONE | www.plosone.org 8 February 2014 | Volume 9 | Issue 2 | e89900
Discussion
The first aim of this meta-analysis was to determine the
effectiveness of different treatment approaches on reading and
spelling performance of reading disabled children and adolescents.
The results reveal that phonics instruction is the most intensively
investigated treatment approach. In addition, it is the only
approach whose effectiveness on reading and spelling performance
in children and adolescents with reading disabilities is statistically
confirmed. This finding is consistent with those reported in
previous meta-analyses [9,45]. At the current state of knowledge, it
is adequate to conclude that the systematic instruction of letter-
sound-correspondences and decoding strategies, and the applica-
tion of these skills in reading and writing activities, is the most
effective method for improving literacy skills of children and
adolescents with reading disabilities. The treatment approach
phonics instruction has not only been evaluated in English-
speaking countries, but also in studies conducted in Spain,
Finland, and Italy. Despite the widespread use of this approach,
it is not yet clear whether these interventions are equally effective
across languages. This question could not be addressed in the
present analysis and needs to be addressed by further research.
Phonics instruction combines elements of reading fluency
training and phonemic awareness training. Reading fluency
trainings emphasize repeated word or text reading practice. The
results of the present meta-analysis suggest that reading fluency
training alone is not an effective way to enhance the reading and
spelling skills of children and adolescents with reading disabilities,
as was reported in a previous meta-analysis [14].
Phonemic awareness trainings are widely recognised as being
effective for the remediation of preschool children at risk for
reading disabilities [46,47]. The present results demonstrate that
when phonemic awareness interventions are provided to school-
aged children and adolescents with reading difficulties, they do not
have a significant effect on a child’s reading or spelling
performance. This indicates that phonemic awareness and reading
fluency trainings alone are not sufficient to achieve substantial
improvements. However, the combination of these two treatment
approaches, represented by phonics instruction, has the potential
to increase the reading and spelling performance of children and
adolescents with reading disabilities.
In terms of reading comprehension training, it was not possible
to confirm a significant influence of this approach on literacy
achievement. This result should be interpreted with caution
because the present meta-analysis included only three comparisons
that evaluated reading comprehension training. All three com-
parisons were conducted by the same author and they demon-
strated negligible [28] to small [26] effect sizes. There is a clear
need to complement these studies with further research.
Figure 5. Funnel plot of standard error by Hedges g for observed and imputed comparisons. Funnel plot displays observed and imputed comparisons evaluating the efficacy of phonics instructions on reading performance. doi:10.1371/journal.pone.0089900.g005
Table 4. Unbiased effect size estimation for the efficacy of phonics instruction on reading performance.
95% CI
Studies trimmed g Lower Upper Q
Observed 0.322 0.177 0.467 26.810
Adjusted 10 0.198 0.039 0.357 50.228
doi:10.1371/journal.pone.0089900.t004
Effectiveness of Interventions for Poor Readers
PLOS ONE | www.plosone.org 9 February 2014 | Volume 9 | Issue 2 | e89900
The mean effect size of coloured lenses (Irlen lenses) did not
reach statistical significance. Some studies compared the effect of
coloured lenses to a placebo control group; other studies used an
untrained control group instead. An interesting observation is that
Irlen lenses showed small effect sizes if the experimental group was
compared to an untreated control group [41]. If the experimental
group was compared to a placebo control group, effect sizes were
negligible [33,41]. This finding confirms earlier systematic reviews
that could not prove any positive effect of coloured lenses on
literacy achievement, and suggests that results are mainly due to
placebo effects [48,49].
Studies that tried to enhance reading and spelling skills of
children and adolescents with reading disabilities by medication
with the nootropic piracetam showed only minor effects, and the
mean effect size for reading performance did not reach statistical
significance. With the possibility of side effects in mind [50] the
risks of medication seem to outweigh its benefits.
Auditory trainings intend to foster reading and spelling by
focussing on the underlying causes of the poor performance. At
first glance, this approach seems convenient, but the results of the
present meta-analysis demonstrate that auditory trainings do not
significantly improve children’s reading and spelling skills. Based
on the results of the present meta-analysis and those reported by
other systematic reviews and non-randomized trials [10,51,52], it
can be concluded that focussing directly on literacy skills is
effective but the efficacy of interventions focussing on the
underlying causes could not be confirmed to date.
The second aim of this meta-analysis was to explore the impact
of various factors on the efficacy of interventions. The results of
subgroup analyses do not allow clear conclusions about what
makes an intervention successful. This may be caused by mutual
confounding in the subgroup analyses, which means that each
moderator could be confounded by any of the other moderators.
This influences the observed association between moderator and
outcomes and distorts the true magnitude of effects. As a
consequence, the results of the performed subgroup analyses
should be interpreted with caution. However, some findings are
worth noting. First, subgroup analyses demonstrated that children
and adolescents with mild reading disabilities show more
improvement in literacy skills than more severely impaired
participants. Second, interventions with higher amounts of
treatment or longer durations of treatment seem to be more
effective in improving literacy skills than therapies with small
amounts of treatment or short-time interventions. Third, consis-
tent with previous meta-analyses [8,14], it was found that
interventions that were conducted by the study author tend to
show higher effect sizes than interventions that were implemented
by other conductors. This suggests that solid and professional
knowledge about reading disability in children and adolescents
might enhance treatment efficacy. Meta-regression or hierarchical
linear methods can be helpful to identify specific variables that
influence the efficacy of an intervention. Due to the small number
of included studies that distinguished or evaluated each variable,
these statistical methods could not be applied in the present meta-
analysis.
Unfortunately, it could not be assessed which intervention is
particularly effective for a specific age or grade level. This was due
to the occurrence that many of the included trials comprised study
subjects of a wide age span. Ever since the meta-analyses of the
NRP in the year 2000 [8], it has been apparent that interventions
are not equally effective for different age groups or grade levels.
Providing children of a wide age span with the same interventions
is therefore not a recommended option for research settings and
clinical practice.
The influence of publication bias was determined with funnel
plots. Publication bias refers to the appearance that many studies
remain unpublished because of negligible effect sizes or non-
significant findings [53]. This is presumably the case in this
research domain. We controlled publication bias exemplarily for
the treatment approach of phonics instructions, but it can be
assumed that this phenomenon is present in the other treatment
approaches as well. Duval and Tweedies trim and fill analysis
estimated and valued the true, unbiased effect size as being small,
but still statistically significant.
Consistent with prior research [9,11,12,14,45], this analysis
demonstrated that severe reading and spelling difficulties can be
ameliorated with appropriate treatment. The need for evidence-
based interventions is obvious given the emotional and academic
consequences for children with persistent reading disorders [6]. To
increase the informative value of studies, research in this domain
should improve its methodological quality. Studies were often
excluded from this analysis because of the absence of randomized
allocation concealment. Randomization tries to secure that known
and unknown determining factors are spread equally across
groups. Research has shown that when meta-analyses include
studies whose allocation concealment is inadequate, effects of
interventions can be misjudged [54]. Each study that was included
in our analysis was randomized, but due to missing methodological
specifications the quality of randomization procedures could not
be determined. An equally important aspect is the assessment of
the dependent variables by a blinded person. It has been
demonstrated [55,56] that effects of interventions are exaggerated
if the relevant outcome measures are not assessed in a blinded test
situation. Therefore, effects can only be attributed to the
conducted intervention if they are observed in a blinded
randomized controlled trial with an adequate concealment
technique. Unfortunately, most of the studies included in the
present meta-analysis did not specify whether the dependent
variable was assessed by a blinded person.
This meta-analysis comprises studies from various English-
speaking and non-English-speaking countries like Finland, Italy,
Spain, and Brazil. To conduct a meaningful meta-analysis with an
adequate number of comparisons, these studies could not be
analyzed separately for different languages or groups of languages.
The transferability of research findings from English-speaking
countries to languages with more consistent orthographies and less
syllabic complexity and vice versa is largely debated [57–59]. It
has been demonstrated that differences between languages affect
children’s literacy acquisition [59,60] and, therefore, it cannot be
generally assumed that symptom based treatment approaches are
equally effective in each language.
The Anglo-American region far outweighs other countries in
quantity and quality of the published work in this research
domain. In order to be able to support children and adolescent
with reading disabilities in different languages with evidence-based
interventions, research in every country has to realign on high-
quality standards. This refers in particular to the intensified
application of blinded randomized controlled trials. Moreover, in
order to solve the questions of the transferability of research
findings across languages, cross-linguistic studies are required.
Supporting Information
Table S1 Study characteristics.
(DOCX)
Checklist S1 PRISMA Checklist.
(DOC)
Effectiveness of Interventions for Poor Readers
PLOS ONE | www.plosone.org 10 February 2014 | Volume 9 | Issue 2 | e89900
Acknowledgments
We would like to thank the authors of the studies included in this review for
responding so willingly to repeated requests for information.
Author Contributions
Analyzed the data: KG. Contributed reagents/materials/analysis tools:
KG KK. Wrote the paper: KG EI. Conceived and designed the meta-
analysis: KG EI KK GSK. Literature search and data extraction: KG.
Contributed critical input: KG EI KK GSK.
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