Determine the probability of disease given the patient s presenting signs, symptoms, characteristics and prevalence of disease. Define prevalence as the frequency of an event in a population. Gather more information usually by performing a diagnostic test .
Some tests reduce uncertainty more than others.
Compare current case to past case. The greater the match between the patients the higher the probability that the cause of disease of the patient is the same as that of the previous patient. Disease is very rare. No previous case to compare with. Events easily remembered may be those which are more recent, dramatic or atypical.