The 2 × 2 calculator turns the four cell counts of an exposure × outcome table into every standard effect measure — RD, RR, OR and NNT with CIs — plus χ² and Fisher's exact p.
For any binary exposure × binary outcome study, the 2 × 2 table contains all the information. Different communities prefer different effect summaries: epidemiology favours RR, case-control studies use OR, clinical trials report RD + NNT. This calculator gives all four so you can match the convention of your field.
When the outcome is common (> 10% incidence), OR overstates RR — this is the famous 'OR ≠ RR' pitfall in observational research. For common outcomes, prefer RR / RD over OR. For rare outcomes, OR ≈ RR.
Four cell counts: a (exposed + event), b (exposed + no event), c (unexposed + event), d (unexposed + no event).
Risk difference, risk ratio, odds ratio, number needed to treat — each with a CI. χ² and Fisher's exact p.
NNT is the number you'd need to treat to prevent one event — but only meaningful when the treatment is genuinely beneficial (RD < 0 when 'event' is bad).
When any cell is 0, OR and RR break down; the calculator adds 0.5 to each cell (Haldane-Anscombe correction) before computing.