The study by Douglas and Smeeth (doi: 10.1136/bmj.a1227) uses the self controlled case series method to study the association between exposure to antipsychotics and the risk of stroke.1 The study found that use of any antipsychotic agent significantly increased the risk of stroke (relative risk 1.73, 95% confidence interval 1.60 to 1.87). The risk of stroke in people with dementia taking any antipsychotic was higher (3.50, 2.97 to 4.12) than in people without dementia taking similar medication (1.41, 1.29 to 1.55).

The self controlled case series method, or case series method for short, can be used to study the association between an acute event and a transient exposure using data only on cases; no separate controls are needed.2

The method uses exposure histories that are retrospectively ascertained in cases to estimate the relative incidence. That is, the incidences of events within risk periods—windows of time during or after experiencing the exposure when people are hypothesised to be at greater risk—relative to the incidences of …