When no samples are available to estimate a probability distribution, we have
to invite some domain experts to evaluate the belief degree that each event
will happen. Perhaps some people think that the belief degree should be
modeled by subjective probability or fuzzy set theory. However, it is usually
inappropriate because both of them may lead to counterintuitive results in
this case. In order to rationally deal with personal belief degrees, uncertainty
theory was founded in 2007 and subsequently studied by many researchers.
Nowadays, uncertainty theory has become a branch of mathematics.