A Unified Approach to a Class of Optimal Selection Problems with an Unknown Number of Options

In the so-called secretary problem, if an unknown number, N, of options arrive at i.i.d. times with a known continuous distribution, then ignorance of how many options there are becomes almost irrelevant: The optimal rule for infinitely many options is shown to be minimax with respect to all possible distributions of N, nearly optimal whenever N is likely to be large, and formal Bayes against a noninformative prior. These results hold whatever the loss function.