Changes

====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 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). Another problem for cross sectional studies is that of '''selection bias''', which may result from a refusal of some individuals to participate.<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.

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====

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.

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, which solves some of the problems described above when investigating prevalence in analytic studies. As for cross sectional studies, failure of some individuals to enrol may result in selection bias, as can individuals dropping out of the study as time progresses ('losses to follow-up'). One other problem with cohort studies is that they can be costly and can take a long period of time to complete.

====Case-control 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.

 

 

===Experimental studies===

===Experimental studies===