Changes

==Calculation of confidence intervals==

==Calculation of confidence intervals==

The calculation of confidence intervals for means or proportions follows the same basic procedure, whereas a slightly different approach is used for other measures such as rates and ratios. Most commonly, confidence intervals will not be calculated manually, but it is useful to know the general approach used.

The calculation of confidence intervals for means or proportions follows the same basic procedure, whereas a slightly different approach is used for other measures such as rates and ratios. Most commonly, confidence intervals will not be calculated manually, but it is useful to know the general approach used. This is based upon the estimation of the '''standard error''' of the parameter in question, which is the the '''standard deviation of the sampling distribution''' of the parameter. That is, if samples of a specified size were taken from the population repeatedly (with replacement after each sampling), the standard deviation of all of these results is the standard error. It is known that with large sample sizes, the sampling distribution will be normally distributed (known as the '''central limit theorem'''), and this distribution can be calculated if the following are known:

As the state of the population is not known, this must be approximated using data from the sample.

===Approach for means and proportions===

===Approach for means and proportions===

The general approach for the calculation of confidence intervals in these cases follows the following steps:  

The general approach for the calculation of confidence intervals in these cases follows the following steps. The precise methods for means and proportions will be described below:  

* Calculate the '''sample standard deviation''', as an approximation of the population standard deviation:

* Calculate the '''sample standard deviation''', as an approximation of the population standard deviation:

* Estimate the '''standard error of the mean''' or proportion by dividing the sample standard deviation by the square root of the number of animals sampled.

* Estimate the '''standard error of the mean''' or proportion by dividing the sample standard deviation by the square root of the number of animals sampled.

* Add the number to the sample mean or proportion to give the '''upper confidence limit'''.

* Add the number to the sample mean or proportion to give the '''upper confidence limit'''.

===Approach for rates and ratios====

===Approach for rates and ratios====

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

[[Category:Veterinary Epidemiology - Statistical Methods|D]]