Changes

==Analytic studies==

==Analytic studies==

Analytical studies are used to test hypotheses by comparison of groups of animals based on their exposure or disease status. Analytical studies can be observational or experimental, in observational studies the experimenter does not apply an intervention to the animals or manipulate their environment they simply observe or take measurements.  

Analytical studies aim to identify different 'subpopulations' of animals (defined by the presence or absence of exposures of interest) amongst which disease experience differs, in an attempt to identify risk factors or protective factors for disease. Depending on the study design, this may be achieved by comparing 'disease outcome' between groups of animals with or without the exposure of interest, or by comparing 'exposure' between groups of animals with or without disease. Analytical studies can be viewed as '''observational''' or '''experimental''' in nature. In the case of observational studies, the investigator does has no control over the exposure status of the animals, whereas in experimental studies, the investigator allocates exposures to a selection of the animals. This has important repercussions for the interpretation of the results, as in the case of observational studies, the groups of animals defined by the exposure of interest may differ from each other in other ways than just the exposure of interest.  

===Observational studies===

===Observational studies===