Although cognitive impairment is described as a core component of the characterization of schizophrenia, a sizable percentage of patients are classified as unimpaired by traditional definitions of impairment. The purpose of this study was to determine the percentage of patients with schizophrenia meeting criteria for a “cognitive function decrement” defined as a current level of cognitive function that falls below the level predicted by premorbid estimates.

Keefe RSE, Eesley CE, Poe M. Defining a cognitive function decrement in schizophrenia.  Biological Psychiatry, 2005; 57(6): 688-691.