  In this exploratory article, we draw attention to the common formal ground
among various estimators such as the belief functions of evidence theory and
their relatives, approximation quality of rough set theory, and contextual
probability. The unifying concept will be a general filter function composed of
a basic probability and a weighting which varies according to the problem at
hand. To compare the various filter functions we conclude with a simulation
study with an example from the area of item response theory.
