Suppose that a random variable X of interest is observed perturbed by independent additive noise Y. This paper concerns the “the least favorable perturbation” Yˆε, which maximizes prediction error E(X−E(X|X+Y))2 in class Y with var(Y)≤ε. We find characterization answer to this question, and show example it can be surprisingly complicated. However, special case where infinitely divisible, soluti...