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This process of sampling from populations poses potential problems, as it must both select a sufficient number of individuals in order to be useful for the purposes of the study (whilst not sampling more than is required), and must also ensure that any biases in the selection process are minimised.

This process of sampling from populations poses potential problems, as it must both select a sufficient number of individuals in order to be useful for the purposes of the study (whilst not sampling more than is required), and must also ensure that any biases in the selection process are minimised.

   

   

==Concepts==

==Populations and samples==

A number of concepts relating to sampling from populations are presented here, using the example of a descriptive study investigating the prevalence of bovine tuberculosis amongst beef cattle in England.  

When sampling from populations and when interpreting the results of studies involving sampling, it is important to consider what can be inferred from the results. A number of concepts are presented here, using the example of a descriptive study investigating the prevalence of bovine tuberculosis amongst beef cattle in England.  

 

===Target population===  

===Target population===  

The target population is the population to which the results of the study may be extrapolated out to, even if not all members of this population were eligible for sampling, and is often not clearly defined. In the example given here, it may be that the target population is viewed as all cattle (beef, dairy and noncommercial) in England, or all beef cattle in Great Britain, or all cattle in Great Britain. The decision regarding which population the results can be extrapolated to will depend on the knowledge and experience of the person interpreting the study, and the suitability of this extrapolation is described as the '''external validity''' of the study.

The target population is the population to which the results of the study may be extrapolated out to, even if not all members of this population were eligible for sampling, and is often not clearly defined. In the example given here, it may be that the target population is viewed as all cattle (beef, dairy and noncommercial) in England, or all beef cattle in Great Britain, or all cattle in Great Britain. The decision regarding which population the results can be extrapolated to will depend on the knowledge and experience of the person interpreting the study, and the suitability of this extrapolation is described as the '''external validity''' of the study.

===Expected variation in the data===

===Expected variation in the data===

The variability of an outcome of interest in the sample collected will have a considerable effect on the precision and power of a study. When the outcome is a continuous variable, this variability can be measured as the variance in the source population. However, in the case of binary outcomes, the concept of variability can be more difficult to comprehend. In these cases, the binomial distribution is used to estimate the variance - calculated as the proportion of animals with the outcome of interest multiplied by the proportion of animals without the outcome of interest. This can be viewed as the expected variation in the proportion estimate of a sample if a number of samples were repeatedly taken from the source population, rather than the variation in the proportion estimate in the source population itself.

The variability of an outcome of interest in the sample collected will have a considerable effect on the precision and power of a study. When the outcome is a continuous variable, this variability can be measured as the variance in the source population. However, in the case of binary outcomes, the concept of variability can be more difficult to comprehend. In these cases, the binomial distribution is used to estimate the variance - calculated as the proportion of animals with the outcome of interest multiplied by the proportion of animals without the outcome of interest. This can be viewed as the expected variation in the proportion estimate of a sample if a number of samples were repeatedly taken from the source population, rather than the variation in the proportion estimate in the source population itself.

[[Category:Veterinary Epidemiology - Introduction|F]]

[[Category:Veterinary Epidemiology - Introduction|F]]