700
edits
Changes
Jump to navigation
Jump to search
no edit summary
==Sample size calculation==
==Sample size calculation==
As mentioned earlier, it is important in any study not only that bias is minimised, but that the sample has sufficient [[Random variation#Precision|precision]] and [[Random variation#Hypothesis testing and study power|power]] (in the case of analytic studies) to answer the question(s) for which the study is intended. Both of these are closely related to the random variability in any sample taken from a population. Although this can be reduced by increasing the sample size, a number of other considerations (usually logistical and economic considerations) will also be acting in order to reduce the number of samples which can realistically be taken. Statistical techniques are therefore available in order to calculate the required sample size. Counterintuitively, these require assumptions to be made regarding the final results of the study, as well as information regarding the required [[Random variation#Confidence intervals|level of confidence]], precision or power of the study. Sample size formulae are not given here, but can be found in most statistical textbooks.
As mentioned earlier, it is important in any study not only that bias is minimised, but that the sample has sufficient [[Random error#Precision|precision]] and [[Random error#Hypothesis testing and study power|power]] (in the case of analytic studies) to answer the question(s) for which the study is intended. Both of these are closely related to the random variability in any sample taken from a population. Although this can be reduced by increasing the sample size, a number of other considerations (usually logistical and economic considerations) will also be acting in order to reduce the number of samples which can realistically be taken. Statistical techniques are therefore available in order to calculate the required sample size. Counterintuitively, these require assumptions to be made regarding the final results of the study, as well as information regarding the required [[Random error#Confidence intervals|level of confidence]], precision or power of the study. Sample size formulae are not given here, but can be found in most statistical textbooks.
===Expected variation in the data===
===Expected variation in the data===
===Level of confidence===
===Level of confidence===
This is used in descriptive studies in order to indicate the level of confidence that the confidence interval of the estimate produced will contain the true population value. Usually, a confidence level of 95% is used. The level of confidence is also The concept of confidence intervals is explained further in the section on [[Random variation]].
This is used in descriptive studies in order to indicate the level of confidence that the confidence interval of the estimate produced will contain the true population value. Usually, a confidence level of 95% is used. The level of confidence is also The concept of confidence intervals is explained further in the section on [[Random error|random variation]].
===Power===
===Power===
===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===
[[Category:Veterinary Epidemiology - General Concepts|F]]
[[Category:Veterinary Epidemiology - General Concepts|G]]