Wiki Sumber Informasi iSIKHNAS

Where the investigation involves multiple farms or a larger area or number of animals and it is not possible to assess and include every animal in a study, it will be necessary to select a subset or sample of cases and controls. If you have a list of non-case animals, then random sampling can be used to select a group of controls. More commonly you will not have a list of all animals to choose from. In this case it may be most practical to select one or more controls for each case from those animals that are close to the case (same paddock or pen) and that meet the definition for a non-case. In an open population (where animals are entering and leaving the local population), controls should be selected from those animals with a similar exposure time to the cases.