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Notation for a general two-by-two table
| exposed | unexposed | sum | |
| cases | p i mi1 | q i mi0 | pimi1+qimi0 |
| noncases | (1-pi)mi1 | (1-qi)mi0 | (1-pi)mi1 + (1-qi)mi0 |
| total | mi1 | mi0 | ni = mi1+mi0 |
| risks | p i | q i | |
| RD | pi-qi | ||
| X i | mi1/n i | ||
| Y i | (pimi1+qimi0)/ni |
Let pi and qi be the risks of the exposed and unexposed in group i. Let mi1 and mi0 be the numbers of exposed and unexposed in group i. RD = risk difference; Xi and Yi are the average exposure and average risk in group i; ni = size of group i.
