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===Probability sampling===
===Probability sampling===
In probability sampling, every individual in the source population has an equal probability of being randomly selected for the study sample. As such, it is the only appropriate method of sampling for descriptive studies (since extrapolation to the source population is of integral importance in these cases). Some types of probability sampling are described below.
In probability sampling, every individual in the source population has a calculable, non-zero probability of being randomly selected for the study sample. As such, it is the '''only''' appropriate method of sampling for descriptive studies (since the ability to extrapolate the results to the source population is of integral importance in these cases). Some types of probability sampling are described below.
====Simple random sampling====
====Simple random sampling====
====Multistage sampling====
====Multistage sampling====
This sampling approach extends the concepts used in cluster sampling in order to avoid sampling every individual within each cluster (since this may be impractical, for example in the case of large sheep farms containing thousands of animals, or there may be very little variation between animals within the clusters). In order to ensure that the probability of selection of each individual (known as a '''secondary sampling unit''') is constant, the sampling of SSUs within the PSUs can either be based on sampling with a '''probability proportional to size''', or through using the same process as described for cluster sampling, with the selection of a set proportion of all animals within each cluster. Sampling with probability proportional to size involves weighting the larger clusters in order to increase their chance of selection, followed by the selection of a set number of animals within each selected cluster. This approach is often used as it is logistically simpler (since the exact same number of animals is sampled, regardless of the size of the farm). Methods of weighting larger clusters are described elsewhere
===Non-probability sampling===
Non probability sampling methods do not use random selection techniques, and so are not ideal from a statistical viewpoint. However, they are commonly used in analytic investigations, where the internal validity does not need to be as high as for descriptive studies.
====Judgement sample====
====Judgement sample====
====Convenience sample====
====Convenience sample====
====Purposive sample====
====Purposive sample====