Sample size required to achieve target confidence of freedom

Input Values

  

Calculate the sample size required to achieve a desired level of confidence of population freedom. Can be applied at the herd level or population level. If applied at the herd level, sensitivity and design prevalence are unit (animal) level values, whereas if applied at the population level, sensitivity and design prevalence are cluster (herd/flock) level values.

Inputs are:

  • the test or herd sensitivity for the sampling;
  • prior confidence of freedom (from previous analyses or opinion);
  • probability of introduction of disease during the surveillance period;
  • the required level of confidence of freedom after the additional surveillance;
  • population size (optional); and
  • the design prevalence as either a proportion or number of units/clusters.

Enter test sensitivity if the target is a herd (cluster) level sensitivity or herd-sensitivity if the target is a system or population level sensitivity. If population size is entered the hypergeometric approximation is used, otherwise the calculation is based on the binomial distribution.

The main outputs are:

  • the the adjusted prior confidence of freedom (taking into account the probability of introduction while the surveillance is being undertaken);
  • the target herd or system sensitivity required to achieve the required confidence of freedom; and
  • the sample size required to achieve the target herd or system sensitivity.
Test (or Herd) Sensitivity:
Prior confidence of freedom:
Probability of introduction during period:
Required confidence of freedom:
Population size (leave blank or 0 if not known):
Design prevalence (proportion or units):
Proportion
Unit(s)

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