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·√(– + –) → [–, –]