The third factor of the WISC-III: it's (probably) not freedom from distractibility.

Objective: This study examined the ecological validity, construct validity, and diagnostic utility of the third factor of the WISC-III, heuristically labeled "Freedom From Distractibility" (FFD).
Method: A sample of 200 children, aged 6 to 11 years, with attention-deficit hyperactivity disorder (ADHD) completed the WISC-III, the Wide Range Achievement Test-Revised, and the Test of Variables of Attention. Objective parent and teacher report measures of attention and hyperactivity were completed.
Results: Mean FFD scores were significantly lower than other WISC-III factor scores. The diagnostic utility of FFD is limited, however, as the majority of these children did not show a significant relative weakness on this index. Correlational analyses failed to support the concurrent, ecological, or construct validity of the FFD. FFD scores were not correlated with a measure of sustained visual attention. Findings suggest that among children with ADHD, a low FFD score may be associated with the presence of a learning disability or poor academic performance. This finding was maintained after level of general intelligence was statistically controlled.
Conclusions: Clinicians and researchers should not view FFD as a reliable or valid index of attention or as a diagnostic screening measure for identifying children with ADHD.