Numbers of false positives to a test

Input values

This utility calculates the expected probability of numbers of false positive results to a test for a given sample size and test specificity, assuming the sample is from an uninfected population. It answers the question: 'For a given sample size and test specificity, and assuming that all animals are uninfected, what is the probability that there will be 1, 2, 3 etc false positives ?'. It also answers the related questions: 'What is the probability of x (1, 2, 3, etc) or more false positive results? and What is the probability of x (1, 2, 3, etc) or less false positive results?'

Inputs required

• Test unit specificity; and
• Sample size.

Outputs

• the likely numbers of false positives (x);
• less than or equal to x false positives; and
• greater than or equal to x false positives.

Formulae

There is no simple formula for these caclulations, which utilise the properties of the binomial probability distribution in R or excel. See example scripts/spreadsheet for details.

References

Resources