  We present the methodology used to measure the single top quark production
cross section in the D0 experiment, and show as an example the results that led
to the first evidence of single top quark production in D0 at the Fermilab
Tevatron proton-antiproton collider. The selected events are mostly
backgrounds, which we separate from the expected signals using three
multivariate analysis techniques, one of them being Bayesian neural networks,
which we will describe here.
