Module 8 formulas

Q: What is the formula for absolute risk (AR)?

A: AR = Number of events / Total number of people in the group.

Q: What is the formula for absolute risk in the exposed group using a 2×2 table?

A: AR = A / (A + B).

Q: What is the formula for absolute risk in the unexposed group using a 2×2 table?

A: AR = C / (C + D).

Q: What is the formula for relative risk (RR)?

A: RR = Risk in exposed (treatment) group / Risk in unexposed (control) group.

Q: What is the formula for RR using a 2×2 table?

A: RR = [A / (A + B)] / [C / (C + D)].

Q: What is the formula for relative risk reduction (RRR)?

A: RRR = 1 − RR.

Q: What is the alternative formula for RRR using risk difference (RD)?

A: RRR = RD / Baseline risk in control group.

Q: What is the formula for risk difference (RD) when measuring excess risk from an exposure?

A: RD = Risk in exposed − Risk in unexposed.

Q: What is the formula for absolute risk reduction (ARR) when measuring treatment benefit?

A: ARR = Risk in control group − Risk in treatment group.

Q: What is the formula for RD using RRR and baseline risk (Rc)?

A: RD ≈ RRR × Rc.

Q: What is the formula for number needed to treat (NNT)?

A: NNT = 1 / ARR (using ARR as a proportion).

Q: What is the formula for number needed to harm (NNH)?

A: NNH = 1 / Absolute increase in risk of harm.

Q: What is the formula for odds ratio (OR)?

A: OR = (A × D) / (B × C).

Q: What is the alternative formula for OR using exposure odds?

A: OR = (A/C) / (B/D).

Q: What is the formula for attributable risk (AR)?

A: AR = Incidence in exposed − Incidence in unexposed.

Q: What is the formula for attributable risk using a 2×2 table?

A: AR = [A / (A + B)] − [C / (C + D)].

Q: What is the formula for population attributable risk (PAR)?

A: PAR = Incidence in total population − Incidence in unexposed population.

Q: What is the formula for the proportion of incidence attributable to exposure in the exposed group?

A: (Incidence in exposed − Incidence in unexposed) / Incidence in exposed.

Q: What is the formula for the proportion of incidence attributable to exposure in the total population?

A: (Incidence in total population − Incidence in unexposed) / Incidence in total population.

RR and OR: Use the _____ group in the numerator when calculating RR, and the cross-products AD/BC for OR.

exposed

• RD vs. ARR: Subtract exposed minus unexposed for excess ____ risk; control minus treatment for _____ benefit.

• exposure;

• treatment

• NNT and NNH: Convert percentages to ____ before taking the reciprocal.

• proportions

• For example, 0.2% = 0.002, so NNT = 1/0.002 = 500.

• RRR: Express as a _____ when interpreting it

• percentage

• (e.g., 0.50 = 50%).

NNT/NNH: Round up to the next whole patient when the result is not an _____.

integer