Study design

Epidemiological studies can be described as belonging to one of two categories: descriptive or analytical. Descriptive studies involve detailed investigations of individuals in order to improve knowledge of disease. Descriptive studies often have no prior hypotheses and are opportunistic studies of disease whereas analytical studies are used to test hypotheses by selection and comparison of groups. However, data obtained from analytical studies can be used in a descriptive manner and vice versa.

Descriptive studies

Descriptive studies include case-series, case-reports and surveys. Although unable to test hypotheses, as they do not involve the comparison of groups, they improve knowledge and understanding of disease and are useful for generating hypotheses.

Case reports

These are descriptions of disease in individual animals (or in a very small number of cases). Although the small number of animals included in these types of studies limit the ability to relate the results to larger populations, they provide useful information for further studies - in particular, in the case of rare or emerging diseases.

Case series

These include greater numbers of individuals than case reports (and can in fact include greater numbers of individuals than surveys in some cases), and therefore provide more information regarding the animal, place, time pattern of disease. However, these studies are often not planned out in advance, and the data collected may have been collected for other reasons than the study in question. This means that the individuals included may not be representative of external populations, and that data on factors of interest may be missing.

Surveys

These are carefully planned studies with clear, specific aims and a defined source population (for example, aiming to estimate the prevalence of disease X in country Y at time Z), which differentiates them from case series studies. Surveys will have a clear sampling strategy and a method of data collection (such as a questionnaire or serological test) used specifically for the study in question. If data regarding both outcomes (such as disease status) and exposures of interest are collected, then the study would be more accurately described as a cross sectional analytic study (see below).

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. 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

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 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).

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