The group-related differences on the measures assessing grammatical skills (i.e., MLU, Sentence Completion, % Complete Sentences, % Paragrammatic Errors) were analyzed with a series of independent-samples t-tests. Also in this case the level of statistical significance was set at p < .0125 (.05/4 dependent variables) after Bonferroni correction for multiple comparisons. Effect sizes were calculated in terms of Cohen’s d (Cohen, 1988). The role played by verbal working memory on linguistic performance was controlled by performing an additional series of One-Way ANCOVAs with the scores at the NWR subtest of the BVL_4-12 as covariate. Table 3 reports the results of these analyses showing the level of group-related significance before(t-tests) and after covariation (ANCOVAs). When verbal working memory was not included in the analysis as covariate, the
independent samples t-tests showed that the group of children with SLI had impaired performance on all of these measures: MLU (t(61) = 2.934; p < .005; d = .74); Sentence Completion (t(62) = 2.679; p < 009; d = .67); % Paragrammatic Errors(t(61) = 3.562; p < .001; d = .89); % Complete Sentences (t(61) = 3.756; p < 001; d = .95). Interestingly, when the analyses were replicated by performing a series of One-Way ANCOVAs with the scores at the NWR subtest of the BVL_4-12 as covariate the group-related differences on almost all of these measures disappeared: MLU (p = .038), Sentence Completion (p = .155),and % Paragrammatic Errors (p = .029)