Robin van Rijthoven

92 Chapter 4 Change in spelling errors due to intervention in children with dyslexia Research question 2a focused on the extent to which children with dyslexia develop their phonological and orthographic spelling due to a phonics through spelling intervention. We first calculated the change per session by subtracting the number of phonological, morphological, and orthographical errors at pre- and posttest. Following Gollwitzer and colleagues (2014), change scores are reliable when standard deviations differ between measurement occasions and there needs tobe a non-zero variation inobserveddifference scores. Our data met these requirements. In order to rule out the effects of variation in length of the intervention and to ensure the analyses had enough statistical power, the individual change score was divided by the number of sessions the intervention for each individual lasted. Paired t-tests with Holm-Bonferroni correction (Holm, 1979) were computed to test the differences between pre- and posttest on overall spelling performance and each type of spelling error, respectively. Results showed an increase in overall spelling scores as well as a decrease in all three error types after intervention (see Table 4.4). Regarding the relative differences in change per session, results showed significant differences between phonological and morphological (t(45) = 4.159, p = .<001, d = 0.46) and morphological and orthographic errors(t(45) = -3.098, p = .003, d = 0.35), and phonological and orthographic errors ((t(45) = 4.613, p < .001, d = 0.10). The change per session is highest for morphologic errors, followed by orthographical and lastly phonological errors. Table 4.4 Descriptive Statistics fromboth Pre- and Posttest on al Dependent Measures, the Results on the T-tests for Paired Samples, Cohen’s d, and the Change per Session. Pretest (n = 52) Posttest (n = 46) Difference scores (t-tests) Change per session M SD M SD t d M SD Word spelling percentiles 5.40 10.50 13.25 15.10 3.41*** 0.60 .26 0.57 Percentage of phonological errors 16.27 25.22 1.96 11.07 -4.34*** 0.73 -.62 0.98 Percentage of morphological errors 40.47 25.64 13.61 16.34 -8.10*** 1.25 -1.05 0.90 Percentage of orthographic errors 20.96 26.18 3.85 12.85 -4.95*** 0.82 -.72 0.99 *p< .05. **p< .01 ***p< .001 Predicting individual differences in change scores in children with dyslexia Finally, research question 2b focused on the predictive role of semantic abilities on the phonological and orthographic spelling development of children with dyslexia due to