Analytical Studies

All epidemiological studies can be divided into observational and experimental studies.

Observational studies observe and measure the effects on disease rates of exposures of interest (as they occur in the population). They are further subdivided in Descriptive and Analytical studies.

Analytical studies
Analytical studies are done in order to find out if an outcome is related to exposure. They can take the form of observational (we don’t influence the population), as well as interventional/experimental (we influence the population, e.g.: clinical trials; we give drugs). There are four main types of analytical studies:

Ecological
The average exposure of a population is compared with the rate of the outcome for that population. The data is obtained for several populations and the data are examined for the evidence of an association between outcome and exposure. The measure of association is usually the correlation coefficient. The unit of analysis is the population, rather than the individual, therefore the only conclusions we can draw relate to the population. There is no possibility to make conclusions about the association between exposure and outcome at the individual level.

Cross-sectional
They measure:
 * 1) Prevalence of exposure together with
 * 2) Prevalence of disease

Information on exposures and disease status are obtained at the same point in time, therefore these studies involve data collection at a defined time. These studies are easy to be conducted and are quick and relatively cheap. They are often used where there is no registry of disease, for example studies of respiratory disease or national health surveys. Cross-sectional studies measure prevalence, not incidence of disease. Temporal sequence of exposure and effect maybe difficult or even impossible to determine. They differ from case-control studies in that they aim to provide data on the entire population under study, whereas case-control studies typically include only individuals with a specific characteristic, with a sample, often a tiny minority, of the rest of the population. Unlike case-control studies, they can be used to describe absolute risks and not only relative risks.

Cohort
Cohort studies start with a group of exposed and a group of unexposed individuals. These groups are then followed up over time and assessed to see who develops the disease. The incidence rate of disease in the exposed group is then compared to that in the unexposed group, therefore measuring relative risk (RR). Since we only select subjects by exposure, we can study several health outcomes at the same time. By properly selecting groups of exposed people we can study relatively rare exposures. Detailed information on confounding factors can be collected allowing control for them either in the analysis or design. The disadvantage of cohort studies is that they can take long time and generally are expensive. For rare diseases, the number of subjects that need to be studied is often so large (since there is a huge majority likely not show the disease), as to make cohort study impractical or unfeasible.

Case-Control
Case-control studies are very useful for rare disease (or other rare health events) where cohort studies would be either difficult or impossible (too large) in order to collect enough events. Case-control studies compare exposures in disease cases vs. healthy controls from the same population. Researchers start with outcome (event/disease) and measure prior exposure in cases and in controls (comparison group). These studies can be used to evaluate many different exposures and are relatively quick to be conducted. The main weakness is that they can look at only one outcome. The reliability of the study depends on the choice of controls. Data are collected retrospectively, therefore they are relatively unreliable. The results are the odds ratio.

Related articles

 * Descriptive Studies