Another classical objective of observational studies is the systematic search for adverse drug reactions (ADR). Due to the restrictions in sample size, phase III clinical trials are at best capable to detect frequent ADRs, mostly by chance, and without providing sufficient data to estimate the underlying incidence rates.
With an observational study, though, it is possible to detect rare ADRs, at least with a pre-specified (high) probability. The incidence rate of a specified ADR describes the frequency at which the ADR occurs in the patient population and is often expressed as, e.g., 0.01% or “1 in 10000 patients”. However, an incidence rate of 0.01% does not imply that in 10000 treated patients exactly 1 specific ADR will occur. The incidence rate is just a statistical expression describing what can be expected with a high probability. Just by chance it could also happen that no event will be observed.
This means: We cannot be sure that the specific ADR will occur in the sample, but we can specify a high probability for the ADR to happen. The higher this probability is, and the more we actually want to be sure, the higher is the resulting sample size.