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Animal and Plant Health Inspection Service
U.S. Department of Agriculture
USDA FAQ's and resources about coronavirus (COVID-19).  LEARN MORE

Premises Sample Size Calculator

It is usually not necessary to sample every at-risk premises in a buffer zone or surveillance zone. Neither is it necessary to surveil all free premises in the free area.  Instead, it is acceptable to randomly sample only a portion of the total number of premises in these zones. This calculator uses an electronic spreadsheet (Excel) to assist the veterinary epidemiologist in estimating the minimum number of premises to sample regardless of whether they are at-risk premises in the buffer zone or surveillance zone or free premises in the free area.

This calculator is used in Section 4: Sampling Plan, Sample Size - Number of Premises to Sample section of the surveillance plan.

How to use the Premises Sample Size Calculator

To find the recommended minimum number of premises to sample to test for disease, Open the Premises Sample Size Calculator and then:

  1. Adjust the confidence level at the top of the calculator, if a level different from the default 95% is desired.
  2. View the sample size matrix and select the column that corresponds to the expected prevalence of disease (.01%-95%) in the population of premises to be sampled. The recommendation is to use 1-2% inter-premises prevalence.
  3. View the sample size matrix and select the row that is equal to (or nearest to) the total premises population (1-100,000) to be sampled.
  4. Read the number listed at the intersection of the selected row and column.

This number corresponds to the minimum number of premises that should be randomly sampled from the total number of premises located in the zone, i.e. at-risk premises if in a buffer zone or surveillance zone and free premises if in the free area.

Tip:Use the output from the calculators as a guide for the sampling plan. The uniqueness of the disease situation, veterinary knowledge, epidemiologic expertise, or other factors such as resource limitations (e.g. monetary, personnel) may indicate a need for sample sizes that are more or less conservative than the estimates provided from this calculator.

About the Premises Sample Size Calculator

The values that populate the cells of the matrix are derived using the  formula:

Number of premises to sample (n)  =  [1-(1-α)1/(p*N)][  N-0.5(p*N)]  + 1
           
Where,

  • p = design prevalence of infection
  • α = confidence level
  • N = number of at-risk and/or free premises in the disease control area/zone
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