  Large differences between the properties of the known sample of cataclysmic
variable stars (CVs) and the predictions of the theory of binary star evolution
have long been recognised. However, because all existing CV samples suffer from
strong selection effects, observational bias must be considered before it is
possible to tell whether there is an inconsistency, which would imply a failure
of the evolutionary model. We have modelled common selection effects and
illustrate their influence on observed CV samples.
