INTERPRETATION OF DIFFERENTIAL DEFICITS - THE CASE OF AGING AND MENTAL ARITHMETIC

A fundamental issue in research on individual differences is the type of evidence sufficient to justify an inference of selective or distinc t deficits in relevant theoretical processes. It is proposed that an i mportant consideration is the extent to which the individual differenc es in 1 variable are independent of those in another variable. Specifi cally, the suggestion presented here is that a strong conclusion of se lective impairment requires evidence that there is significant group-r elated variance in 1 variable after the variance in the other relevant variable is controlled. Furthermore, an inference that the groups are equivalent on a particular theoretical process requires evidence that the variable presumed to reflect that process has sufficient unique v ariance to justify the claim that a distinct process is being assessed . The proposed methods are illustrated with two studies comparing adul ts of different ages in mental arithmetic tasks.