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 - Odds Ratio
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Statistical Measures

Relative Risk & Odds Ratio

Odds Ratio

The formula for OR is:

Odds Ratio = ad / bc

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.