Odds Ratio vs Relative Risk

	Disease Present (Cases)	Disease Absent (Controls)	Row Total
Exposed	a	b	a + b
Not Exposed	c	d	c + d
Column Total	a + c	b + d	Grand Total

• Relative Risk (RR) compares PROBABILITY.

  • (a/a+c) is the probability of exposure in one group.
  • It’s a ratio of two probabilities.

• Odds Ratio (OR) compares ODDS.

  • Odds = (Number of times an event happens) / (Number of times it doesn’t).
  • (a/c) are the odds of exposure in the disease group (cases).
  • (b/d) are the odds of exposure in the control group.
  • The OR is the ratio of these two odds.