Question 1

When can I use relative risk vs. odds ratio?

Accepted Answer

Relative risk can be directly calculated from cohort studies, randomized controlled trials, and cross-sectional studies. In case-control studies, only the odds ratio can be calculated because the sampling design does not preserve the incidence rate. When the outcome is rare (< 10%), OR and RR are approximately equal.

Question 2

What is the Number Needed to Treat (NNT)?

Accepted Answer

NNT is the reciprocal of the absolute risk difference. It tells you how many patients need to receive the treatment for one additional patient to benefit. For example, NNT = 20 means you need to treat 20 people for 1 person to benefit. Lower NNT = more effective treatment.

Question 3

Why is absolute risk difference important alongside relative risk?

Accepted Answer

A large relative risk can be misleading if the baseline risk is tiny. A treatment reducing risk from 0.002% to 0.001% has RR = 2 (impressive sounding) but only reduces absolute risk by 0.001% (NNT = 100,000). Always consider both relative and absolute measures.

Relative Risk Calculator

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