The principle of additivity and its relation to clinical decision making

Clients (N = 178) with varying degrees of organic indicators were administered a psychological battery including the Memory for Designs Test (MFD; Graham & Kendall, 1960) to assess the extent to which joint presence of organic indicators affected the sensitivity of the MFD in identifying organicity. Joint presence failed to add to the discriminability of the MFD, although individual instruments such as the PIAT (Arithmetic) and WAIS-R (PIQ less than VIQ) clearly discriminated across the MFD.