Less unique variance than meets the eye: overlap among traditional neuropsychological dimensions in schizophrenia. Schizophr Bull

Themagnitude of the overlap among dimensions of neuropsy-chological testperformance inschizophreniahasbeenthesub-ject of perennial controversy. This issue has taken on renewed importance with the recent focus on cognition as a treatment target in schizophrenia. A substantial body of factor analytic literature indicates that dimensions are separable in schizo-phrenia. However, this literature is generally uninformative as to whether the separable dimensions are independent, weakly correlated, or strongly correlated. Factor analyses have often used methods (ie, principal components analysis with orthogonal rotation) that preclude this determination, and correlations among factor-based domain composites and underlying measures have been reported infrequently in these studies. Current meta-analyses of reported ‘‘between-dimension’ ’ correlations for individual neuropsychological measures and for cognitive domain composite variables indi-cate that cognition variables in schizophrenia are correlated, on average, at a ‘‘medium’ ’ level of r 5 0.37 for individual measures from different cognitive dimensions and r 5 0.45 fordomaincomposites.Because these aremeanbivariate cor-relations, the multiple correlation of an individual measure with all the other measures in a cognitive battery is likely to be higher. Measure reliabilities of 0.80 or less also imply greater commonality among traditional neuropsychological measures. In short, there are underappreciated constraints on the amount of reliable cognitive performance variance in traditional neuropsychological test batteries that is free to vary independently.Theabilityof suchbatteries toreveal cog-nitivedomain–specific treatmenteffects inschizophreniamay be much more limited than is generally assumed