computePValue {FFD}

FUNCTION to compute the probability of finding no testpositives in a sample of a certain size.

Description

For a population of size nPopulation with a given design prevalence the function computes the probability of finding no testpositives in a sample of size nSample if an imperfect test is used (given sensitivity and specificity). This probability corresponds to the alpha-error (=error of the first kind) of the overall test with null hypothesis: prevalence = design prevalence. A modified hypergeometric formula is used; see Cameron, Baldock, 1998.

Usage

computePValue(nPopulation, nSample, nDiseased, 
    sensitivity, specificity = 1)

Arguments

nPopulation

Integer. Population size.

nSample

Integer. Size of sample.

nDiseased

Integer. Number of diseased elements in the population according to the design prevalence.

sensitivity

Numeric between 0 and 1. Sensitivity (= probability of a testpositive result, given the tested individual is diseased) of the test (e.g., diagnostic test or herd test).

specificity

Numeric between 0 and 1. Specificity (= probability of a testnegative result, given the tested individual is not diseased) of the test (e.g., diagnostic test or herd test). The default value is 1.

Value

The return value is a numeric between 0 and 1. It is the probability of finding no testpositives (not diseased!) in the sample.

Author(s)

Ian Kopacka <[e-mail]>

References

A.R. Cameron and F.C. Baldock, "A new probablility formula to substantiate freedom from disease", Prev. Vet. Med. 34 (1998), pp. 1-17.

See Also

computeOptimalSampleSize, computeAlphaLimitedSampling

Examples

alphaError <- computePValue(nPopulation = 3000, 
    nSample = 1387, nDiseased = 6, sensitivity = 0.85, specificity = 1)