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==Descriptive studies==

==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.

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. The overarching aims of descriptive studies are covered in [[Descriptive epidemiological studies|another page]].

====Case reports====

====Case reports====

==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. 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.  

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 not have 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===

====Case-control studies====

====Case-control studies====

Case-control studies are based upon the identification of two populations of individuals - one ('cases') comprising those who have experienced the outcome of interest (e.g. disease) and one ('controls') comprising those who have not experienced the outcome of interest (but who are otherwise comparable to the cases). As the outcome itself is involved in the selection of participants, no measures of disease frequency can be made from a case-control study, which are only used for the identification of exposures associated with disease (through the estimation of 'odds ratios'). Although case-control studies can be very useful in the investigation of rare diseases, there can be considerable difficulties in ensuring that the control group are comparable to the case group. If these groups are not comparable, there can be considerable selection bias, which an invalidate the results of the study.

Case-control studies are based upon the identification of two populations of individuals - one ('cases') comprising those who have experienced the outcome of interest (e.g. disease) and one ('controls') comprising those who have not experienced the outcome of interest (but who are otherwise comparable to the cases). As the outcome itself is involved in the selection of participants, no measures of disease frequency can be made from a case-control study, which are only used for the identification of exposures associated with disease (through the estimation of 'odds ratios'). Although case-control studies can be very useful in the investigation of rare diseases, there can be considerable difficulties in ensuring that the control group are comparable to the case group. If these groups are not comparable, there can be considerable selection bias, which can invalidate the results of the study.

===Experimental studies===

===Experimental studies===

In experimental studies (also known as '''intervention studies'''), the investigator allocates the exposure of interest to a selection of the participants prior to following them up. The allocation of the exposure should be randomised, and there should be a clear control group which does not receive the exposure (known as a '''Randomised Controlled Trial'''). Ideally, 'blinding' of participants and investigators to the treatment allocation should also be performed whenever possible in order to reduce any differences between the groups. As biases and confounding are reduced through the randomised allocation of exposure, these studies provide the best quality of evidence of any single study type. However, in a similar fashion to cohort studies, they can be very costly, both in terms of money and time. Additionally, in the case of suspected harmful exposures, randomised controlled trials may not be an ethical option, and so they are commonly used when investigating interventions which are suspected to be beneficial.

In experimental studies (also known as '''intervention studies'''), the investigator allocates the exposure of interest to a selection of the participants prior to following them up. The allocation of the exposure should be randomised, and there should be a clear control group which does not receive the exposure (known as a '''Randomised Controlled Trial'''). Ideally, 'blinding' of participants and investigators to the treatment allocation should also be performed whenever possible in order to reduce any differences between the groups. As biases and confounding are reduced through the randomised allocation of exposure, these studies provide the best quality of evidence of any single study type. However, in a similar fashion to cohort studies, they can be very costly, both in terms of money and time. Additionally, in the case of suspected harmful exposures, randomised controlled trials may not be an ethical option, and so they are commonly used when investigating interventions which are suspected to be beneficial.