The four methods of diagnosis

1. Pattern recognition - as for Downs' facies or psoriasis.

2. Algorithm - such as the work-up for polycythaemia. It must contain every possibility.

3. Inductive method - exhaustion. The student method of performing a complete history or examination is not often used by experienced clinicians except to buy thinking time.

4. Hypothetico-deductive method - educated guessing plus testing. A short list of the three to six most likely diagnoses is explored by questioning, examination and investigations and the list revised until minimal acceptable uncertainty is achieved. This is the usual approach of experienced practitioners towards most diseases not instantly recognised.

ref: The Oxford T/B of Med 3e ed DJ Weatherall et al 1996