Create a 2×2 table:
Exposed with outcome = a
Exposed without outcome = b
Unexposed with outcome = c
Unexposed without outcome = d
Use the formula:
Odds Ratio = (a × d) / (b × c)
Or calculate odds in each group:
Odds in exposed group = a / b
Odds in unexposed group = c / d
Odds Ratio = (a / b) ÷ (c / d)
Interpret the result:
OR = 1 means no association
OR > 1 means higher odds with exposure
OR < 1 means lower odds with exposure