  In this paper the following selection problem is discussed. A set of $n$
items is given and we wish to choose a subset of exactly $p$ items of the
minimum total cost. This problem is a special case of 0-1 knapsack in which all
the item weights are equal to~1. Its deterministic version has a trivial
$O(n)$-time algorithm, which consists in choosing $p$ items of the smallest
costs. In this paper it is assumed that the item costs are uncertain. Two
robust models, namely two-stage and recoverable ones, under discrete and
interval uncertainty representations, are discussed. Several positive and
negative complexity results for both of them are provided.
