School-Based Health Center Use and High School Dropout Rates—Reply

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Treating time-varying variables as fixed can result in a systematic bias referred to as “time-dependent bias,”¹ “immortal time bias,”² or “survival bias.”³ This is a common analytic error; in a review of publications from 9 top medical journals from 1998 to 2002, of 127 studies that used survival analysis with time-dependent variables, 41% (52 studies) were susceptible to time-dependent bias because they did not appropriately operationalize at least 1 time-varying variable in the model.¹