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| Block 1: Epidemiology |
| EPI4-4: THE ODDS RATIO:LINK WITH THE OTHER MEASURES OF EFFECT |
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| Block 1: Epidemiology |
| EPI4-4: THE ODDS RATIO:LINK WITH THE OTHER MEASURES OF EFFECT |
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SESSION OBJECTIVE |
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At the end of this session you should understand the link between the Odds Ratio and other measures of effect - the prevalence ratio, the incidence proportion ratio and the incidence rate ratio. |
The relationship between the Odds Ratio and the Prevalence Ratio or Incidence Proportion Ration is simpler to understand.
| D+ | D- | ||
| E+ | a | c | M1 |
| E- | b | d | M0 |
| N1 | N1 | T |
If the data are based on a prevalence or an incidence study in a closed cohort,the OR can also be worked out a third way.
The prevalence ratio or the incidence proportion ratio of disease in the exposed to that in the unexposed is (a/M1) / (b/M0),
which is equal to (a/a + c)/(b/b + d).
If the prevalence (a+b)/T is small (usually less than 10%) , it can be assumed that a and b are small in relation to c and d and then (schematically)
(a + c) ~ c and (b + d) ~ d,
and the above prevalence ratio is approximately (~) ad/bc which is the Odds Ratio.
| D+ | D- | ||
|---|---|---|---|
| E+ | 53 | 53 | 106 |
| E- | 43 | 85 | 128 |
| 96 | 138 | 234 |
If the data come from either type of study rather than a case control study the Prevalence Ratio or the Incidence Proportion Ratio would be 53 x 128/106 x 43 = 1.49, which is close to the OR of 1.89.
