Box 2. Calculating prevalence measures of occurrence in the GATE frame

 *Only percentages and means presented in HERS paper.3 Prevalence =  (number of people with an outcome the number in a population) at time T Exposure group occurrence [EGO] or experimental event rate [EER] =  a ÷ E at time T  =  a ÷ (a+c) at time T Example: Prevalence of smoking in HRT group at time of randomisation EGO (or EER)  =  13 smokers per 100 women (13%)* Example: Average HDL cholesterol concentrations in HRT group at 1 year EGO =  sum of HDL cholesterol concentrations in the women in HRT group ÷ the number of women in HRT group  =  1.40 mmol/l* Comparison group occurrence [CGO] or control event rate [CER] =  b ÷ C at time T  =  b ÷ (b+d) at time T Example: Prevalence of smoking in placebo group at time of randomisation CGO (or CER)  =  13 smokers per 100 women (13%)* Example: Mean HDL cholesterol concentrations in placebo group at 1 year CGO =  sum of HDL cholesterol concentrations in the women in placebo group ÷ the number of women in placebo group  =  1.27 mmol/l*