Box 3.

Randomisation of patients before data are available to confirm their eligibility

A clinical trial is designed to study the effectiveness of a new anti-influenza drug. To be effective the drug must be given within 48 hours of onset of influenza symptoms, which may be indistinguishable from symptoms of other infections

All consenting patients who present to a doctor within 48 hours of development of influenza-like symptoms are enrolled and randomised into the trial

The study protocol stipulates that only patients who later give positive results on culture or serological tests for influenza infection will be analysed in the results, meaning that a number of patients randomised to the study are excluded retrospectively