Question 1

What is the difference between odds ratio and relative risk?

Accepted Answer

Odds ratio (OR) compares the odds of an event between groups: OR = (a×d)/(b×c). Relative risk (RR) compares event probabilities: RR = (a/(a+b))/(c/(c+d)). OR is always valid for both cohort and case-control designs. RR is only valid for cohort/RCT designs where you can estimate true incidence. When the event rate is low (<10%), OR $\approx$ RR. At higher event rates, OR overestimates RR.

Question 2

What is NNT?

Accepted Answer

Number Needed to Treat (NNT) is the reciprocal of the absolute risk reduction: NNT = 1/ARR. It answers: "How many patients must be treated with the experimental intervention for one additional patient to benefit?" NNT = 10 means you treat 10 patients to prevent one event. When the exposure increases risk, the reciprocal is called NNH (Number Needed to Harm).

Question 3

When should I use OR vs RR?

Accepted Answer

Use RR for cohort studies and RCTs — it is directly interpretable as a ratio of risks. Use OR for case-control studies — it is the only valid measure because you cannot estimate true incidence from a case-control design. In meta-analyses, OR is often preferred because it has better mathematical properties (symmetry, no boundaries issues). Always report the measure appropriate to your study design.

Question 4

What happens when a cell is zero?

Accepted Answer

A zero cell makes the OR or RR undefined (0 or ∞). The Haldane-Anscombe correction adds 0.5 to all four cells, producing finite estimates with valid confidence intervals. This correction is standard in meta-analysis. The tool applies it automatically when enabled.

Question 5

How are confidence intervals computed?

Accepted Answer

OR and RR confidence intervals use the log-scale normal approximation: ln(OR) $\pm$ z $	imes$ SE(ln(OR)), then exponentiated. ARR uses a normal approximation to the difference of proportions. NNT CIs are the reciprocals of the ARR CI bounds (which may include infinity if the ARR CI crosses zero).

OR / RR / NNT Calculator

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