Instructions for the Premises Sample Size Table

Last Modified: February 07, 2024

It is not always possible or even necessary to sample every premises of a zone or area. Randomly selecting a subset of the total number of premises often meets the surveillance objectives. 

(By “total number of premises in a zone or area,” we mean the total number of premises belonging to the inference group in the zone or area.) The Premises Sample Size Table provides sample sizes for a range of the total number of premises in the zone or area and for a range of prevalence thresholds among premises in that zone or area.

How to Use the Premises Sample Size Table

To find the recommended minimum number of premises to select to test for disease, open the Premises Sample Size Table and then:

  1. Adjust the Confidence Level if a level different from the default 95% is desired.
  2. Adjust the Probability of Detection within the Premises (based on the number of animals tested). Enter the Confidence Level used in the Animal Sample Size Table. If the number of animals recommended by the Animal Sample Size Table was not sampled, estimate the Probability of Detection within the Premises by decreasing the Confidence Level in the Animal Sample Size Table until the cell in the table shows the number of animals actually sampled.
  3. Select the column corresponding to the Prevalence Threshold of Disease (0.01%–95.00%) in the Premises Sample Size Table required to meet the surveillance objectives. One recommendation is to use 1–2% inter-premises prevalence. Note that 2% of 100 premises means that there are 2 infected premises in this zone or area, while 2% of 1,000 means there are 20 infected premises in the zone or area.
  4. Select the row equal to (or nearest to) the total Number of Premises in the Zone or Area (1–100,000) to be sampled. Round up to be conservative.
  5. 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 or area to achieve the desired probability of detection when the probability of detection for each premises is the value entered.

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 a sample size that differs from the one provided by this sample size table. If a deviation from the sample size given in the table must be made, keep the Confidence Level at 95% or above and note the Prevalence Threshold of Disease the affordable sample size provides. Consider the consequences of missing disease below that new threshold.

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