Читать книгу Medical Statistics - David Machin - Страница 91
The Odd Ratios are Symmetrical but the Relative Risk Is Not
ОглавлениеConsider the data in the 2 × 2 contingency table of Table 3.7 where the relative risk of being alive in the exposed compared to the not exposed group is relative risk (alive) = 0.96 /0 99 = 0.97; the reciprocal is 1/relative risk (alive) = 1/ 0.97 = 1.03. The relative risk (dead) = 0.04/0.01 = 4. Thus, note that the relative risk (dead) is not equal to 1 / relative risk (alive).
Table 3.7 Example of a two by two contingency table with a binary outcome (alive or dead) and two groups of subjects (exposed or not exposed).
| Outcome | Test treatment exposed | Control treatment not exposed |
|---|---|---|
| Alive | 0.96 | 0.99 |
| Dead | 0.04 | 0.01 |
| Total | 1.00 | 1.00 |
The odds ratio (alive) = (0.96/0.04) / (0.99/0.01) = 0.24; the reciprocal is: 1/odds ratio (alive) = 1/ 0.24 = 4.13. The odds ratio (dead) = (0.04/0.96)/(0.01/0.99) = 4.13; and hence the odds ratio (dead) is equal to 1/odds ratio (alive).
