USMLE STEP 3 BIOSTATS: FORMULAS WITH COMPLETE SOLUTIONS

2 x 2 table Correct Answer Top vs Side: Disease vs Exposure Reality vs Test Result A,B,C,D (Left to Right, Top to Bottom) Sensitivity Correct Answer A/(A+C) or TP/(TP+FN) Specificity Correct Answer D/(D+B) or TN/(TN+FP) PPV Correct Answer A/(A+B) or TP/ (TP+FP) NPV Correct Answer D/(D+C) or TN/ (TN+FN) Relative Risk Correct Answer A/(A+B) / C/(C+D) Use in Cohort Remember: In RR you are looking forward at risk of a disease/outcome in the *exposed vs unexposed groups* *risk of disease/outcome* in exposed vs. *risk of disease/outcome* in non-exposed RR comparing groups Correct Answer If the RR of an outcome in group A as compared to group B is x then the RR in group B as compared to group A is 1/x Odds Ratio Correct Answer (A/C)/(B/D) or AD/BC Use in Case Control Remember: Odds of *exposure* in the cases divided by Odds of *exposure* in the controls (because you are looking back in time). So it's not like in RR where you are looking forward at *risk of disease/outcome* (*risk of disease/outcome* in exposed vs. *risk of disease/outcome* in non-exposed)