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==Analytic studies==
==Analytic studies==
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.
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. The ultimate aim is to draw conclusions regarding possible causative associations between exposures and disease (although, as mentioned earlier, causation is impossible to prove). 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===
As mentioned above, observational studies are based on the investigator observing the real-life situation and drawing inferences from this. Therefore, there is potential for biases and confounding, which must be considered when interpreting the results. Observational studies can be classified as one of three types, according to the method of selection of participants (although some studies may use aspects of different study designs). The study design will affect which measures of disease are possible.
====Cross sectional studies====
====Cross sectional studies====
Cross sectional studies involve the selection of a sample of the population, regardless of their exposure or outcome status. As the sample is collected at one point in time, the '''prevalence''' of disease can be estimated, and this must be considered when identifying associations. As the prevalence of disease at any one point in time is dependent upon both the incidence of disease and the duration of disease, this can cause problems when trying to identify causal associations - primarily because the prerequisite for causation stating that the exposure must precede the outcome may not be able to be definitively proved (as the exposure may have been different at the time the animal actually developed the disease).<br>
Cross sectional approaches can also be used to follow up a population over time, by repeatedly sampling from the population - known as a '''repeated cross sectional''' design. Although this may appear to be similar to a '''cohort study''' (see below), they differ in that in a repeated cross sectional study, the same individual animals are not necessarily sampled each time, and so are not followed up over time.
====Cohort studies====
Cohort studies, as mentioned above, involve following animals over time in order to record whether or not they experience the outcome of interest. The selection of animals may be based upon exposure status (in which case, the study design is a cohort study, sensu stricto), or a selection of disease-negative animals may be made, with the exposure status determined after selection (which is strictly known as a longitudinal study). Cohort studies allow the measurement of the '''incidence''' of disease, as all animals are negative at the start of the study and are then followed up over time.
====Case-control studies====
====Case-control studies====
Case-control studies
===Experimental studies===
===Experimental studies===