Secondary population proxy : exclusion criteria group. We created an additional population proxy comparison group based on appendicitis group assignment exclusion criteria 3 and 4 listed above. This exclusion criteria comparison group was created as
follows: 1) we selected all individuals who were not assigned to any of the alcohol or drug cohorts as potentially eligible for the exclusion criteria comparison group; 2) we randomly selected one admission per individual in this initial group and designated the selected admission as the index admission; 3) to be eligible for the exclusion criteria comparison group, individuals could not have any prior or concurrent indication (in relation to their index admission) of drug use (see Table 1 for ICD-9 codes) or schizophrenia or schizophrenia-related conditions (as defined previously); and 4) eligible individuals could not have any drug episodes prior to or concurrent with their schizophrenia outcome or the end of the study period, whichever occurred first.