We consider a new model for computing with uncertainty. It is desired to compute a function fX1; : : : ; Xn where X1; : : : ; Xn are unknown, but guaranteed to lie in speci ed intervals I1; : : : ; In . It is possible to query the precise value of any Xj at a cost cj. The goal is to pin down the value of f to within a precision at a minimum possible cost. We focus on the selection function f which returns the value of the kth smallest argument. We present optimal o ine and online algorithms for this problem.