USMLE purpose: Prepare for “guaranteed” board-style biostats questions by recognizing the high-frequency patterns quickly.

How to use this page

  1. Memorize the core pattern table.
  2. Drill the formulas until you can write them without hesitation.
  3. Use the practice question toggles to confirm recall.
  4. Treat this as a rapid review page before a question block.

One-liner

Most “guaranteed” biostats items are not conceptually hard; they test whether you can recognize the metric and avoid the wrong denominator.

High-frequency pattern table

Question wording Metric Formula / move
Positive test, actually disease? PPV TP / (TP + FP)
Negative test, actually no disease? NPV TN / (TN + FN)
Disease present, test positive? Sensitivity TP / (TP + FN)
Disease absent, test negative? Specificity TN / (TN + FP)
Exposure followed to outcome Relative risk Risk exposed / risk unexposed
Disease vs no disease, look back Odds ratio AD / BC

Common traps

⚠️ Trap: Using sensitivity when the question asks about a positive test result.

Fix: If the test result is already known, think predictive value.

⚠️ Trap: Putting the total sample size in every denominator.

Fix: Use the denominator that matches the subgroup being asked about.

Anki-style rapid recall