  We introduce an index for measuring the influence of the k-th smallest
variable on a pseudo-Boolean function. This index is defined from a weighted
least squares approximation of the function by linear combinations of order
statistic functions. We give explicit expressions for both the index and the
approximation and discuss some properties of the index. Finally, we show that
this index subsumes the concept of system signature in engineering reliability
and that of cardinality index in decision making.
