There is also concern about whether the primary population proxy appendicitis comparison group was appropriately chosen. However, as mentioned previously, the unmatched appendicitis group in our study manifested an incidence rate of schizophrenia similar to that in a recent review of literature. In addition, the appendicitis group and our secondary population proxy group had similar incidence rates of schizophrenia, and analyses using either comparison group produced similar results. To address the potential ascertainment bias that might have occurred if comparison subjects had a different propensity to visit a hospital that would provide a psychiatric diagnosis, we
matched cohorts on number of hospital admissions.