Changes

====Stratified random sampling====

====Stratified random sampling====

This form of sampling is based on simple or systematic random techniques, but prior to selection of the study sample, the source population is divided into a number of strata (often according to factors considered to be associated with disease). Most commonly, the proportion of animals within each stratum in the source population is used as the proportion of the total sample size to be taken from each stratum (and therefore, the number of animals to be selected per stratum). This approach ensures that every animal has an equal probability of selection.

This form of sampling is based on simple or systematic random techniques, but prior to selection of the study sample, the source population is divided into a number of strata (often according to factors considered to be associated with disease). Most commonly, the proportion of animals within each stratum in the source population is used as the proportion of the total sample size to be taken from each stratum (and therefore, the number of animals to be selected per stratum). This approach ensures that every animal has an equal probability of selection. However, other approaches may be used which produce a 'weighted sample' (for example, animals from one particular stratum may be oversampled) - it is important to note that even in these cases, the sampling strategy is still a probability sample, as the probability for each animal within each stratum can still be calculated (even if the probability differs between strata). In these cases, additional approaches must be applied in the analysis stage in order to 'unweight' the sample.

====Cluster sampling====

====Cluster sampling====

Cluster sampling is used in cases where the individual animals of interest are 'clustered' within other groupings (such as animals within farms), and it is easier to sample many animals from a smaller number of clusters than it would be to sample small numbers of animals from many clusters (as would be the likely situation if simple random sampling was used), or if a sampling frame of the clusters (known as the '''primary sampling units''') but not the individual animals is available. A random sample of clusters is first made (using simple or systematic random sampling techniques), followed by sampling of every individual within the selected cluster. As each cluster has an equal probability of being selected, and as every animal within these clusters is then sampled, the probability of selection of any individual animal is constant.

Cluster sampling is used in cases where the individual animals of interest are 'clustered' within other groupings (such as animals within farms), and it is easier to sample many animals from a smaller number of clusters than it would be to sample small numbers of animals from many clusters (as would be the likely situation if simple random sampling was used), or if a sampling frame of the clusters (known as the '''primary sampling units''') but not the individual animals is available. A random sample of clusters is first made (using simple or systematic random sampling techniques), followed by sampling of every individual within the selected cluster. As each cluster has an equal probability of being selected, and as every animal within these clusters is then sampled, the probability of selection of any individual animal is constant. It should however be noted that variation in the outcome of interest is likely to be lower within clusters than between clusters, meaning that this must be accounted for when calculating the sample size and when interpreting the results.

 

 

===Non-probability sampling===

===Non-probability sampling===