To identify the IPAS items that are most relevant for characterizing aggressive behavior in substance dependent individuals, an exploratory PCA was conducted on the IPAS using the current sample. Consistent with the prior factor analytic studies (Haden et al., in press; Kockler et al., 2006; Stanford et al., 2003), we first conducted an item analysis in which item-total correlations were calculated for each scale and t tests were computed between item scores in the top and bottom quartiles of the respective scale to determine whether items differentiated extreme groups. Four items did not have a significant corrected total item correlation (p<.05) or did not differentiate between extreme groups and were dropped from further analysis. The remaining 26 items were submitted to the PCA. The number of factors to be rotated was determined by the use of tables in Lautenschlager (1989). A varimax rotation with .40 as the minimum loading criterion was used to obtain an orthogonal factor solution. One item (loading <.40) was excluded and three factors were identified. Based on the content of the items, the scales were identified
