Occam's razor (also known as the KISS principle or lex parsimoniae), an often cited principle of parsimony, economy, or succinctness used in problem-solving.
It states that among competing hypotheses, the one with the fewest assumptions should be selected. Although other, more complicated solutions may ultimately prove correct, in the absence of certainty the fewer assumptions made, the better.
Who does not remember university, where loads of students were (and constantly are) told "When you hear hoofbeats, look for horses, not zebras," which means look for the simplest, most common explanation first.
A more useful approach to me is thinking of probability of the diagnosis in general with decreasing probability down the list:
- typical manifestation of a common disorder
- atypical manifestation of a common disorder
- typical manifestation of a rare disorder
- atypical manifestation of a rare disorder
The other way round:
Often, suggesting rare diseases will rarely prove right.