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====Convenience sampling====
====Convenience sampling====
The selection of the study population in these cases is based on convenience - for example, ease of access, availability of data or the presence of a sampling frame. Despite the limitations of this approach, the criteria for selection can often be clearly defined, and an analytic study using convenience sampling may be able to be extrapolated to a larger target population in some cases.
The selection of the study sample in these cases is based on convenience - for example, ease of access, availability of data or the presence of a sampling frame. Despite the limitations of this approach, the criteria for selection can often be clearly defined, and an analytic study using convenience sampling may be able to be extrapolated to a larger target population in some cases.
====Purposive sampling====
====Purposive sampling====
This sampling strategy involves the inclusion of a subset of the total population in the sample - for example, only including infected or exposed animals. It can therefore be viewed as a change in the definition of the source population - meaning that if random sampling techniques are used in order to select the study sample, it is actually a probability sample involving only a subset of the population.
==Sample size calculation==
==Sample size calculation==