After the pioneering works by Robbins {1944, 1945) and Choquet (1955),
the notation of a set-valued random variable (called a random closed set
in literatures) was systematically introduced by Kendall {1974) and
Matheron {1975). It is well known that the theory of set-valued random
variables is a natural extension of that of general real-valued random
variables or random vectors. However, owing to the topological structure
of the space of closed sets and special features of set-theoretic
operations ( cf. Beer [27]), set-valued random variables have many
special properties. This gives new meanings for the classical
probability theory. As a result of the development in this area in the
past more than 30 years, the theory of set-valued random variables with
many applications has become one of new and active branches in
probability theory. In practice also, we are often faced with random
experiments whose outcomes are not numbers but are expressed in inexact
linguistic terms.