computeAlpha: FUNCTION to compute the herd-based alpha-errors (= 1 - herd...
In FFD: Freedom from Disease

Description Usage Arguments Value Author(s) References See Also Examples

For a vector of herd sizes the herd-based alpha-errors (= 1-herd sensitivity) are computed for either limited or individual sampling; see Ziller et al.

1 2	computeAlpha(nAnimalVec, method, sampleSizeLtd, herdSensitivity, intraHerdPrevalence, diagSensitivity, diagSpecificity = 1)

`nAnimalVec`	Integer vector. Stock sizes of the herds.
`method`	Character string. `"individual"` for individual sampling or `"limited"` for limited sampling.
`sampleSizeLtd`	Integer. Required only if `method == "limited"`. Sample size for limited sampling, i.e., for each herd `sampleSizeLtd` animals are tested, or of the herd contains less than `sampleSizeLtd` animals the entire herd is tested.
`herdSensitivity`	Numeric between 0 and 1. Required only if `method == "individual"`. Desired (minimal) hed sensitivity for individual sampling. The number of animals to test per herd is determined according to that value.
`intraHerdPrevalence`	Numeric between 0 and 1. Intra-herd prevalence. The number of diseased animals per herd is computed as `max(1,round(intraHerdPrevalence*nAnimalVec))` (it is assumed that at least one animal is diseased).
`diagSensitivity`	Numeric between 0 and 1. Sensitivity (= probability of a testpositive result, given the tested individual is diseased) of the diagnostic test.
`diagSpecificity`	Numeric between 0 and 1. Specificity (= probability of a testnegative result, given the tested individual is not diseased) of the diagnostic test. The default value is 1, i.e., perfect specificity, and is recommended.

Returns a vector containing the herd-based alpha-errors, where each entry in the vector corresponds to an entry in the input argument nAnimalVec.

Ian Kopacka <ian.kopacka@ages.at>

M. Ziller, T. Selhorst, J. Teuffert, M. Kramer and H. Schlueter, "Analysis of sampling strategies to substantiate freedom from disease in large areas", Prev. Vet. Med. 52 (2002), pp. 333-343.

Is used in the method sample for classes IndSampling and LtdSampling.

data(sheepData)
## Compute the herd sensitivities usinh limited sampling:
alphaVec <- computeAlpha(nAnimalVec = sheepData$nSheep, 
    method = "limited", sampleSizeLtd = 7, 
    intraHerdPrevalence = 0.2, diagSensitivity = 0.9)