Risk Calculator — Relative Risk, Odds Ratio & NNT
Enter the four cells of a 2×2 epidemiological table — exposed vs. unexposed and event vs. no event — to instantly calculate Relative Risk, Odds Ratio, Absolute Risk Difference, Number Needed to Treat (or Harm), and Population Attributable Risk.
Exposure increases risk (RR > 1)
- 1
Risk in exposed group
30 ÷ (30 + 70) = 30 ÷ 100 = 0,3 - 2
Risk in unexposed group
10 ÷ (10 + 90) = 10 ÷ 100 = 0,1 - 3
Relative Risk (RR)
0,3 ÷ 0,1 = 3RR > 1 means exposure increases risk; RR < 1 means it is protective.
Jak tato kalkulačka funguje?
Enter a 2×2 table (a = exposed+event, b = exposed+no-event, c = unexposed+event, d = unexposed+no-event). RR = [a/(a+b)] / [c/(c+d)], OR = (a×d)/(b×c), NNT = 1/|ARD|. RR > 1 means exposure increases risk; RR < 1 is protective.
Vzorec
How this is calculated
The 2×2 contingency table is the workhorse of epidemiology and clinical research. Rows represent exposure status (exposed vs. unexposed, or treatment vs. control) and columns represent whether the outcome event occurred. Cell a: exposed with event; b: exposed without event; c: unexposed with event; d: unexposed without event.
The incidence risk in each group is simply the number of events divided by the group size: risk_exposed = a / (a+b) and risk_unexposed = c / (c+d). The Relative Risk (also called Risk Ratio, RR) divides these two: RR = risk_exposed / risk_unexposed. An RR of 2 means the exposed group is twice as likely to experience the event as the unexposed group. An RR less than 1 signals a protective exposure; RR = 1 means no difference.
The Odds Ratio (OR) uses odds rather than probabilities: OR = (a×d)/(b×c). The OR approximates the RR when the event is rare (below ~10% in both groups) — a fact known as the rare-disease assumption. For common outcomes the OR diverges from RR and should not be interpreted as a risk ratio. The Absolute Risk Difference (ARD) is the arithmetic difference in risks; its reciprocal is the Number Needed to Treat (NNT) — how many people need to be exposed to see one additional event — or the Number Needed to Harm (NNH) if exposure increases risk.
Často kladené otázky
Relative Risk is the natural measure for cohort studies where you directly observe incidence. Odds Ratio is the only valid measure from case-control studies (where you sample by outcome, not exposure). When the event rate is below about 10%, the OR approximates the RR closely. For common outcomes in a cohort study, prefer RR.
Number Needed to Treat (NNT) is the reciprocal of the Absolute Risk Reduction: NNT = 1 / |ARD|. It answers: "how many patients must receive the treatment for one additional patient to benefit?" An NNT of 5 is very effective; an NNT of 100 is modest. When exposure increases risk, the same formula gives Number Needed to Harm (NNH).
PAR (or etiologic fraction) estimates the proportion of total cases in the population that can be attributed to the exposure: PAR = (total incidence − background risk) / total incidence. It is useful for public-health decisions about the impact of eliminating a risk factor from the entire population.
Také známé jako
TG we-Calculate Editorial Team. (2026). Risk Calculator — Relative Risk, Odds Ratio & NNT [Online calculator]. TG we-Calculate. https://we-calculate.com/cs/calculator/risk-calculator
TG we-Calculate Editorial Team. "Risk Calculator — Relative Risk, Odds Ratio & NNT." TG we-Calculate. 2026. https://we-calculate.com/cs/calculator/risk-calculator.
TG we-Calculate Editorial Team, "Risk Calculator — Relative Risk, Odds Ratio & NNT," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/cs/calculator/risk-calculator
@misc{wecalculate_risk_calculator, title = {Risk Calculator — Relative Risk, Odds Ratio & NNT}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/cs/calculator/risk-calculator}}, year = {2026}, note = {TG we-Calculate} }
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