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===Clustering===
===Clustering===
When cluster or multistage sampling techniques are used, the effect of clustering of the outcome of interest within clusters will have an effect on the required sample size, since animals within the same cluster would be expected to be more similar to each other than to those from other clusters. Therefore, formulas are available in order to calculate the 'design effect' (or DEFF), which indicates the factor by which the calculated sample size needs to be increased by in order to account for this.
When cluster or multistage sampling techniques are used, the effect of clustering of the outcome of interest within clusters will have an effect on the required sample size, since animals within the same cluster would be expected to be more similar to each other than to those from other clusters. Therefore, formulas are available in order to calculate the 'design effect' (or DEFF), which indicates the factor by which the calculated sample size needs to be increased by in order to account for this.
===Sampling fraction===
This relates to the proportion of the total target population which is sampled. In most epidemiological studies, samples are collected '''without replacement''' (i.e. an individual animal cannot be selected twice), although many of the calculations used are based on the concept of sampling with replacement. This does not cause a problem when (as in most cases), the sampling fraction is low (less than about 5%, when expressed as a percentage). However, if the sampling fraction is high, a correction known as the '''finite population correction''' should be made to account for this in the calculation of the required sample size (and in the final estimates).
===Multivariable studies===
===Multivariable studies===