It is widely accepted that deficits in phonological awareness skills are related to reading difficulties. Recently, another source of reading difficulty has been identified that involves naming speed, and combined impairments in phonological skills and naming speed will produce more severe reading deficits than single deficits in either of these cognitive skills. The purpose of this study was to investigate the consequences of grouping children based on the presence or absence of deficits in these skills. We demonstrate that the greater severity of reading impairment found in children with a double deficit could be due in part to a statistical artifact caused by grouping children based on their performance on two correlated continuous variables. This artifact also makes it difficult to establish the relative impact of deficits in naming speed on reading ability independent of deficits in phonological awareness.
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