M3 · Measures of effect

Effect measures from a 2×2 table

Enter your own counts and read the risk ratio, odds ratio, risk difference and NNT side by side — each translated into plain language.

InterventionComparatorOutcome

Event

No event

Intervention

Comparator

Risk (intervention) = — · Risk (comparator) = —

Risk ratio (RR)

—

95% CI —

Odds ratio (OR)

—

95% CI —

Risk difference (RD)

—

95% CI —

Number needed to treat (NNT)

—

95% CI —

Read outcome as:

Enter all four counts to see results.

How it's computed

Inputs a = – b = – c = – d = –

Risks Ri = a/(a+b) = –/– = –

Rc = c/(c+d) = –/– = –

Risk ratio RR = Ri/Rc = –/– = –

Odds ratio OR = (a·d)/(b·c) = (–·–)/(–·–) = –

Risk diff RD = Ri − Rc = – − – = –

NNT NNT = 1/|RD| = 1/– = –

Show confidence-interval maths

RR: ln(RR) ± 1.96·√(1/a − 1/(a+b) + 1/c − 1/(c+d)) = ln(–) ± 1.96·√(1/– − 1/– + 1/– − 1/–) → [–, –]

OR: ln(OR) ± 1.96·√(1/a + 1/b + 1/c + 1/d) = ln(–) ± 1.96·√(1/– + 1/– + 1/– + 1/–) → [–, –]

RD: RD ± 1.96·√(Ri(1−Ri)/(a+b) + Rc(1−Rc)/(c+d)) = – ± 1.96·√(– + –) → [–, –]