Abstract
When conducting a clinical study, a "normal" control population is often desired. Identifying normal patients in a clinical data repository (CDR) can be challenging because healthy patients do not go the hospital; and, because patients receive care from multiple hospitals, the absence of a diagnosis in one hospital's electronic health record does not mean a patient does not have the disease. We define a set of 10 simple heuristic filters to eliminate patients who would seemingly be poor candidates for a normal control (e.g., chronic conditions, rare diseases, no recent data, etc.). Surprisingly, out of 2,019,774 patients at two large academic hospitals, these filters excluded all but 31,352 (1.55%). This illustrates how difficult it can be to identify control cohorts, and it raises questions of what it truly means to be normal.