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* | |