Table C-1. Herd Subset Sampling

Number of Cattle in Herd (2nd or higher lactation)

Number of Cattle to Sample (2nd or higher lactation)

 

ELISA

Fecal culture

<300

Test all

Test all

301–400

Test all

313

401–500

Test all

324

501–600

531

332

601–700

540

338

701–800

547

342

801–900

552

345

µ 901

580

360

Notes: In smaller herds, all cattle second or higher lactation must be tested. In herds with fewer than 30 second- and higher-lactation animals, first-lactation animals must also be tested. Sample numbers above have been based on the following assumptions:

• The cattle to be tested are in second or higher lactation.

• For these calculations, ELISA tests was assumed to have 25% sensitivity and fecal cultures was assumed to have 40% sensitivity (These were consensus estimates of the Herd Status Committee of the NJWG, USAHA for sub-clinically infected cows in first or higher lactation, and no changes were made for older populations sampled.)

• For these calculations, 100% test specificity of the ELISA and fecal culture was assumed (given follow-up of all ELISA positives with fecal culture).

• The confidence of detecting infection (at least 1 test-positive cow), if present at a true prevalence of 2%, is 95%.

• Sampling without replacement.



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