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Attrition Adjusted Sample Calculator

Work out attrition adjusted sample instantly with clear inputs, formula shown and shareable results.

About Attrition Adjusted Sample Calculator

Sample size must be inflated for both attrition and screening failure, and the two compound. Needing 200 at analysis with 20% attrition means enrolling 250, and a 15% screen failure rate means screening 295 - an inflation factor of 1.47. Planning only for attrition would leave the study 45 participants short.

Formulas

Enrolment target

enrol = analysable target / (1 - attrition)

Screening target

screen = enrolment target / (1 - screen failure rate)

Worked example

With the default values below, Attrition Adjusted Sample Calculator returns Participants To Enrol of 250. Change any input above and every figure here is recalculated in your browser.

Inputs used

Participants needed at analysisminimum 1
200
Expected attritionaccepts 0–90 %
20 %
Screening failure rateaccepts 0–90 %
15 %

Results

Participants To Enrol
250
Participants To Screen
295
Recruitment Inflation Factor
1.471 x
Expected Dropouts
50

Step by step

  1. 1Values used
  2. 2Enrolment target
  3. 3Screening target
  4. 4Participants To Enrol= 250
  5. 5Participants To Screen= 295
  6. 6Recruitment Inflation Factor= 1.471 x
  7. 7Expected Dropouts= 50

Assumptions and limits

  • The result is derived only from the 3 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

Should I use intention-to-treat instead of inflating?

Intention-to-treat analyses everyone randomised, but missing outcome data still reduces effective power, so inflation is still needed.

What attrition should I assume?

It varies hugely: 5-10% for a short single-visit study, 20-30% for a year-long trial, and more for hard-to-reach populations.

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