USMLE purpose: Use the 2×2 table to calculate PPV, specificity, standard deviations, and correlation direction quickly.

How to use this page

  1. Start with the diagnostic-test box and make sure you can label TP, FP, FN, and TN.
  2. Review the core formulas until you can choose the right formula from the wording of the stem.
  3. Work through the PSA example without looking at the answer.
  4. Do the practice questions before opening the answer toggles.
  5. Use the transcript only if you want to verify the original video wording.

One-liner

This episode is a rapid drill on the highest-yield “box math” moves: build the 2×2 table from prevalence, sensitivity, and specificity; calculate PPV and specificity; apply standard deviation cutoffs; and interpret correlation direction.

Why this matters for USMLE

🎯 If the stem gives sensitivity, specificity, prevalence, or a 2×2 table, slow down and draw the box. These questions are usually free points if the table is labeled correctly.

Core formulas / rules

Concept Formula / rule Exam trigger
PPV TP / (TP + FP) “Positive test; probability patient truly has disease?”
Specificity TN / (TN + FP) “Correctly identifies disease-free patients”
Prevalence effect Higher prevalence → higher PPV; lower prevalence → lower PPV Screening test performance in different populations
Standard deviation Mean ±1 SD = 68%; ±2 SD = 95%; ±3 SD = 99.7% Normally distributed value above/below a cutoff
Correlation coefficient Direction = sign; tightness = magnitude Scatter plot interpretation

High-yield visual: diagnostic-test box

Disease + Disease −
Test + True positive False positive
Test − False negative True negative

Episode-specific notes

1. Positive predictive value from prevalence, sensitivity, and specificity