Clinical audit is both a part of clinical governance and an essential component of infection prevention and control. It is frequently performed on a proportion of the target population. The sample should represent the source population and be sufficient for statistical analysis. In a hospital, infection control practices are likely to be quite similar within the same clinical area (cluster effect). This must be taken into consideration when calculating the necessary number of patients. Sample size is determined by the desired level of precision for estimating the compliance rate, or by the difference between observed and expected rates, or on the difference before and after implementation of interventions. To estimate the hospital-wide compliance rate without additional costs we suggest focusing the audit on a large number of wards, even if fewer observations within each ward are obtained, rather than auditing a large number of practices on a restricted number of wards.