avoid chasing zebras

The first is exemplified by the advice given to medical students that they should ‘avoid chasing zebras’. If a patient’s symptoms can be explained by the hypothesis that she has common disease C, and also can be explained by the hypothesis that she has rare disease R, you should prefer the C diagnosis over the R. C is said to be more parsimonious. In this case, the more parsimonious hypothesis has the higher probability of being true.