even if studies are perfectly designed and carried out, the results may show variability because of the play of chance. A confidence interval covers the likely range of the true effect. For example, the result of a study may be that 40 per cent (95 per cent confidence interval 30–50 per cent) of people are helped by a treatment. That means that we can be 95 per cent certain that the true effect is between 30 and 50 per cent.