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 Module IV Home Objectives Using Statistical Measures Frequency Distributions Ratios, Proportions, Rates Morbidity Frequency Measures Mortality Frequency Measures Relative Risk & Odds Ratio Outbreak Exercises Quiz Outline

# Statistical Measures

## Relative Risk & Odds Ratio

### Odds Ratio

The formula for OR is:

Odds Ratio = ad / bc

 Where a =    number of persons with disease and with exposure of interest b =    number of persons without disease, but with exposure of interest c =    number of persons with disease, but without exposure of interest d =    number of persons without disease and without exposure of interest a+c =    total number of persons with disease (“cases”) b+d =    total number of persons without disease (“controls”)

The OR in this example is:

Odds Ratio = (46 x 40) / (25 x 18)   =   1840 / 450   =   4.1

So those who became ill were 4.1 times as likely to have eaten the tuna casserole. We should probably look a little more deeply into the tuna casserole! We would still need to subject this result to a test of statistical significance (just like we do with the RR) to judge the probability that the result could have occurred by chance alone.