THE EFFECTS OF THE Z-SCORE TRANSFORMATION ON MEASURES OF RELATIVE ERECTILE RESPONSE STRENGTH - A REAPPRAISAL

In studies of stimulus control of sexual arousal, some researchers tra nsform raw data to z-scores. Earls, Quinsey and Castonguay [Archives o f Sexual Behavior, 16, 493 500 (1987)] have argued that the proportion of variance due to stimulus presentations was greater in z-scores tha n in the raw data or in a percent score. We present analyses of sexual arousal data that show that z-score transformations may distort the i nformation inherent in the raw data and may increase random error. In addition, we present Monte Carlo analyses indicating that the z-score transformation compromises the estimates of Type I error, and, dependi ng on specific circumstances, sometimes increases and sometimes decrea ses the power of the statistical test. Transformation of raw data to p ercent of full erection does not distort the information in the raw da ta, does not compromise the data analyses and, therefore, is the prefe rred transformation.