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Placebo Response Rate Calculator

Work out placebo response rate instantly with clear inputs, formula shown and shareable results.

About Placebo Response Rate Calculator

The placebo response rate is often substantial - 32% here - and only the difference above it counts as drug effect. A 58% treatment response gives 26 percentage points of absolute benefit, a number needed to treat of 3.8. Reporting only the relative ratio of 1.81 obscures how much of the response placebo already delivered.

Formulas

Absolute benefit

difference = treatment response rate - placebo response rate

Number needed to treat

NNT = 1 / absolute risk difference

Worked example

With the default values below, Placebo Response Rate Calculator returns Placebo Response Rate of 32.00%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Responders in the placebo armminimum 0
32
Placebo arm sizeminimum 1
100
Responders in the treatment armminimum 0
58
Treatment arm sizeminimum 1
100

Results

Placebo Response Rate
32.00%
Absolute Benefit Over Placebo
26.00%
Number Needed To Treat
3.8
Response Rate Ratio
1.813

Step by step

  1. 1Values used
  2. 2Absolute benefit
  3. 3Number needed to treat
  4. 4Placebo Response Rate= 32.00
  5. 5Absolute Benefit Over Placebo= 26.00
  6. 6Number Needed To Treat= 3.8
  7. 7Response Rate Ratio= 1.813

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why are placebo responses so high?

Natural history, regression to the mean, expectation effects and the attention of trial participation all contribute. In depression trials placebo response often exceeds 30%.

Which measure should be reported?

Absolute difference and NNT, because relative measures can look impressive while representing a tiny absolute change.

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