Pennsylvania Animal Diagnostic Laboratory System
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How Many Animals Do I Need to Test?

You must have a Java enhanced browser or you won't get a calculated answer.

This equation is from Dr. David Galligan's BaseCow spreadsheet application

This function calculates the sample sized needed to detect at least one infected animal at the significance level entered with a test having a sensitivity value.

Reference: Noordhuizen et al, Application of Quantitative Methods in Veterinary Epidemiology. 1997 Wageningen. Page 50

Example application: If a herd of 100 cows is suspected of having Johne's at about 5%, how many animals should you test to be 95% confident that the herd is free of Johnes. (Elisa test sensitivity = 45%)

The inputs are:

• Population number: 100
• Prevalence: 0.05
• Significance: 0.95
• Test sensitivity: 0.45
• Answer (rounded to nearest integer): 73

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