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.
