Doubling is not decay: The shelf life of medical knowledge

To the Editor, Eva's paper examining the myth persuasively shows that the claim that medical knowledge doubles every 73 days rests on no identifiable data and that PubMed publication counts instead suggest a doubling time of roughly a decade. 1 We agree that panic is a poor basis for curricular reform.However, the paper's reassurance may extend further than its data allow.Eva's analysis measures accumulation: how quickly the stock of publications grows.Yet the conclusion turns to obsolescence, stating that 'we need not worry about what we teach in medical schools being obsolete within months of graduation'. 1 Doubling and decay are different quantities.A literature can grow slowly while some of what it asserts is overturned quickly, and publication counts cannot tell us how long any given claim remains valid.Survival analyses of decay itself paint a heterogeneous picture.