Mr. Conor Geraghty:

The variation really occurs depending on the time of the disease. Very early on in the disease, one might miss all of them but those animals are still there for the following tests and that is when one sees the disease showing up because it is further on. If one is unlucky enough just to hit it earlier on in the disease outbreak, one may miss some animals but as time goes on, it will become more apparent. Animals are not all infected at the same time. On those maps which show one animal getting infected, then infecting another and another, there is a timespan. Generally, it is highly effective at identifying infected herds but when one goes back to do the retest, one might take out two or three more, if that makes sense.