A central concept in any epidemiological investigation is that of appropriate data description. A number of methods are available for describing data, and the selection of the incorrect one can result in a loss of information, or more seriously, a misleading estimate. The most appropriate measure to use will depend on the type of data in question.
Qualitative data
Qualitative data may or may not have an intrinsic order, and can always be described using proportions (i.e. the proportion of animals in each 'category'). The mode can also be a useful measure of central tendency, in the case of ordinal data. There are no meaningful measures of spread in these cases, as the difference between adjacent categories is not standard.
Quantitative data
These data can be described according to a measure of central tendency, spread and the shape of their distribution.