Examination of all animals at the time of collection of samples allows an estimate to be made of the earliest date on which clinical disease first appeared. From this, and knowledge of the longest and shortest incubation periods, it is possible to estimate the earliest and latest possible dates of introduction of infection onto the farm. This is useful to target possible movements "onto" the farm that might have been responsible for introducing infection (tracing backwards). It is also possible to estimate the earliest date from which virus is likely to have been produced and therefore the period for which it is most urgent to trace movements "off" (tracing forwards).