Authors should provide up to three typical risks for participants receiving the control intervention. It is recommended that these be presented in the form of a number of people experiencing the event per 1000 people (natural frequency). A suitable alternative greater than 1000 may be used for rare events, or 100 may be used for more frequent events. Assumed control intervention risks could be based on assessments of typical risks in different patient groups or at different lengths of follow-up. Ideally, risks would reflect groups that clinicians can easily identify on the basis of their presenting features. A footnote should specify the source or rationale for each control group risk, including the time period to which it corresponds where appropriate. In Figure 11.5.a, clinicians can easily differentiate individuals with risk factors for deep venous thrombosis from those without. If there is known to be little variation in baseline risk then review authors may use the median control group risk across studies.