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Animal and Plant Health Inspection Service
U.S. Department of Agriculture
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Interval Sampling Calculator

As the name implies, interval sampling involves systematically sampling premises (or animals) at set intervals from among the population to be sampled. For example, suppose the veterinary epidemiologist wants to draw a sample of 100 premises from a population of 1,200 premises (or, alternatively, 100 animals from a herd of 1,200 animals). The first step in implementing interval sampling is to divide the population of 1,200 by the sample size of 100. The sampling interval is 12 (1,200/100). The next step is to draw a random number, either manually or via automation, which is less than or equal to the sampling interval of 12. Let’s say the random number selected was 10. Thus, every 10th, 20th, 30th, etc. multiple of 10 premises or animals would be selected for sampling until the sample size of 100 premises or animals is selected.  

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

How to use the Interval Sampling Calculator

Tip: Read ALL instructions BEFORE using! Failure to do so may cause your computer to freeze.

  1. Open the Interval Sampling Calculator.
  2. To use the Interval Sampling Calculator, whether it opens in Excel or in a web browser, one Excel setting must be updated. The directions are provided for Excel 2010.
    • Step 1: If the calculator opens in a web browser, you must save a copy of the calculator to your computer, then open the calculator in Excel.
    • Step 2: Enable Analysis Toolpak Addin: Click the File tab, and then click Options. Click Add-Ins, and then in the Manage box, select Excel Add-ins. Click Go. In the Add-Ins available box, select the Analysis ToolPak check box, and then click OK. Tip:  If Analysis ToolPak is not listed in the Add-Ins available box, click Browse to locate it. If you get prompted that the Analysis ToolPak is not currently installed on your computer, click Yes to install it.
  3. Update the spreadsheet by entering the sample size and the total population of premises or animals from which the sample size is to be drawn.
  4. The calculator should automatically list by number the specific premises or animals to sample
  5. Double check that the list of premises or animals generated by the calculator (1) begins with a premises or animal number which is less than or equal to the sampling interval and (2) the count of specific premises or animals listed by the calculator equals the sample size inputted into the calculator. If either one or both of these values are incorrect, this calculator is not operating properly.

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