All medical research is carried out on selected individuals, although the selection criteria are not always clear. The usefulness of research lies primarily in the generalisation of the findings rather than in the information gained about those particular individuals. We study the patients in a trial not to find out anything about them but to predict what might happen to future patients given these treatments.

A recent randomised trial showed no benefit of fine needle aspiration over expectant management in women with simple ovarian cysts.1 The clinical question is whether the results can be deemed to apply to a given patient. For most conditions it is widely accepted that a finding like this validly predicts the effect of treatment in other hospitals and in other countries. It would not, however, be safe to make predictions about patients with another condition, such as a breast lump. In between these extremes lie some cases where generalisability is less clear.

For example, when trials showed the benefits of β blockers after myocardial infarction the studies had been …