USMLE purpose: Master relative-risk table setup: convert a word problem into a 2×2 table, then calculate the requested comparison.

How to use this page

  1. Draw the 2×2 table before doing math.
  2. Put exposure/treatment across the top and outcome down the side.
  3. For relative risk, use risk in exposed ÷ risk in unexposed.
  4. Use the transcript only if you want the original walkthrough.

One-liner

Relative risk is risk in the exposed/treatment group divided by risk in the unexposed/control group; the hard part is putting the numbers in the correct 2×2 boxes.

2×2 table setup

Outcome + Outcome - Total
Exposure + A B A + B
Exposure - C D C + D

Core formulas

Metric Formula When to use
Risk in exposed A / (A + B) Event rate among exposed
Risk in unexposed C / (C + D) Event rate among unexposed
Relative risk [A / (A + B)] ÷ [C / (C + D)] Cohort/RCT-style risk comparison
Odds ratio AD / BC Case-control-style association

Board Exam Buzzwords

Phrase What to do Trap
“Out of 400 exposed, 100 developed outcome” 100 is outcome+; 300 is outcome- Do not put 400 into a single cell
“Compared to” First group / second group Reversing numerator and denominator
Relative risk One cell over row/column total divided by another risk Using odds ratio by accident

Anki-style rapid recall

Practice Questions