USMLE Step 3 Biostats: Formulas Questions And Answers.

USMLE Step 3 Biostats: Formulas
2 x 2 table - Top vs Side:
Disease vs Exposure
Reality vs Test Result
A,B,C,D (Left to Right, Top to Bottom)

Sensitivity - A/(A+C)
or TP/(TP+FN)

Specificity - D/(D+B)
or TN/(TN+FP)

PPV - A/(A+B)
or TP/ (TP+FP)

NPV - D/(D+C)
or TN/ (TN+FN)

Relative Risk - 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 - 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 - (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)

Absolute Risk Reduction (ARR) (used when evaluating an intervention/treatment) -
Event Rate in Control - Event Rate in Exposed

aka:

CER - EER