Relationships Between Cognitive Abilities and Reading Skills: Testing for Nonlinearity

Cognitive and academic assessment are fundamental to learning disability diagnosis in children and adults. This study sought to evaluate the relationships between cognitive abilities and reading skills in a sample of referred postsecondary students. It was hypothesized that the relationship between cognitive ability and reading skills may change at different points in the ability distribution, such that the relationship between the two variables is strong and linear up until a certain point and then the relationship weakens or becomes non-significant. The study assessed relationships between four empirically supported indices of cognitive ability and eight measures of reading skills (a total of 32 combinations of relationships). First, linear relationships were analyzed. Strong linear relationships were found for each of the variable combinations, consistent with prior research on non-referred and referred child samples. Next, curvilinear regression was used to see if data were better fit using a curved line (evidence of a threshold effect). There were generally null effects for added variance, though there may be some evidence for a curvilinear relationship between cognitive ability and the specific skill of decoding. These regressions were also re-run while controlling for demographic variables, and again a third time when individuals with possible response bias (i.e., low effort or malingering) were removed; the results did not change. Finally, linear correlations were calculated separately for students in different parts of the distribution of cognitive ability. Again, results indicated that the relationships remain linear across the distribution of cognitive ability. While our primary hypothesis was not supported, this study extended previous research to better understand the linear nature of cognitive-achievement relationships among referred adults. Implications for practice are nuanced and depend on which model of learning disability diagnosis is used