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 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.