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Audit Sample Size Calculator

Work out audit sample size instantly with clear inputs, formula shown and shareable results.

About Audit Sample Size Calculator

Attribute sampling for control testing sizes the sample from the confidence level and the precision, which is the gap between the tolerable and expected error rates. A narrow gap demands a large sample, which is why an auditor expecting 4 percent errors against a 5 percent tolerance needs far more items than one expecting zero. With no expected errors, zero-acceptance sampling gives a much smaller sample.

Formula

Attribute sampling

precision = tolerable - expected; n = z^2 p(1-p) / precision^2; with zero expected errors n = ln(1 - confidence) / ln(1 - tolerable); apply the finite-population correction

Worked example

With the default values below, Audit Sample Size Calculator returns Items to test of 24. Change any input above and every figure here is recalculated in your browser.

Inputs used

Population of transactions or recordsminimum 1
24,000
Confidence levelaccepts 80–99.9 %
95 %
Tolerable error rateaccepts 0.1–50 %
5 %
Expected error rateaccepts 0–49 %
1 %

Results

Items to test
24
Deviations allowed before the control fails
0
Share of the population tested
0.100%
Allowance for sampling risk
4.00%

Step by step

  1. 1Values used
  2. 2Attribute sampling
  3. 3Items to test= 24
  4. 4Deviations allowed before the control fails= 0
  5. 5Share of the population tested= 0.100
  6. 6Allowance for sampling risk= 4.00

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 does a low tolerable error rate cost so much sample?

Sample size scales with the inverse square of precision. Halving the tolerable rate roughly quadruples the items to test.

What if a deviation is found beyond the allowance?

The control is assessed as ineffective for the period. Expanding the sample after the fact is generally not acceptable, since it biases the conclusion.

Related Calculators

Audit Sample Size Calculator – Calculate Items to test