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===Study sample===
===Study sample===
The sample population includes those animals which are included in the final study. It is important to remember that in most epidemiological studies, we are not interested in this population ''per se'' - rather, we are interested in using this sample as a proxy for the source population (and possibly the target population). Because not all members of the source population have been sampled, statistical techniques need to be applied to the results from the study group in order to estimate what the characteristics of the source population are expected to be. Due to this extrapolation, there is always a possibility that any estimates from a sample are incorrect due to '''random variation''' in the sample. Although this random variation cannot be controlled without increasing the sample size (or redefining the source population), the accuracy of the estimate can be maximised by ensuring that sources of [[Bias|'''bias''']] are minimised.
The sample population includes those animals which are included in the final study. It is important to remember that in most epidemiological studies, we are not interested in this population ''per se'' - rather, we are interested in using this sample in order to make statements regarding the source population (and possibly the target population). Because not all members of the source population have been sampled, statistical techniques need to be applied to the results from the study group in order to estimate what the characteristics of the source population are expected to be. Due to this extrapolation, there is always a possibility that any estimates from a sample are incorrect due to '''random variation''' in the sample. Although this random variation cannot be controlled without increasing the sample size (or redefining the source population), the accuracy of the estimate can be maximised by ensuring that sources of [[Bias|'''bias''']] are minimised.
===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.
====Simple random sampling====
====Systematic random sampling====
====Stratified random sampling====
====Cluster sampling====
===Non-probability sampling===
===Non-probability sampling===