This was done in several steps. First, exploratory factor analysis (EFA) was performed separately in all of the sites that utilized the OPCRIT (Operational Criteria for Psychotic Illness) (25), PANSS (Positive and Negative Syndrome Scale)(26) as follows. For each dataset (i.e. one instrument from each individual site) individual items were excluded if they had > 50% missing data. Remaining missing data was inferred using the method of multiple imputation, as operationalized in Proc Mi in SAS, resulting in five separately imputed datasets for each input dataset. For each item, we used the mean of the corresponding imputed items from these five datasets. These final input variables were entered into EFA using principal component analysis, implemented in SAS using Proc Factor, using VARIMAX rotation (SAS Institute, Cary, N.C.).
