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As mentioned in the introduction to the Methods section, the theoretically ideal study design would be a prospective cohort study. This design is generally not feasible for several reasons described in this section. A number of prospective studies have followed a cohort of multiple sclerosis(MS) patients to evaluate other health outcomes such as other autoimmune diseases, cancer, and severity of disease (life expectancy, disability, and clinical course). But, to our knowledge, no study so far could be classified as a true prospective study of a cohort of nondiseased individuals with the goal of examining the risk factors for development of the disease.
The most important feature of the cohort design is that exposure is measured and recorded before disease outcome is known. An association found using such a design is more likely to be interpreted as a causal relationship. Although a true prospective cohort study is not suitable for studying MS, there is a practical variant of this design that includes this important characteristic. This is the historical cohort design described below.
This design takes advantage of individual information on putative risk factors already collected for other purposes. Such information may be found in registers or files assembled for a variety of reasons. Examples are vaccinations, childhood infections, and other infections from school health records and from military enrollment files, antibody status in serum registries, occupation and sociodemographic data from population censuses, and work-related exposure in files from labor unions. The individuals included in such registers will compose the cohort or study population. These individuals, who were free of disease when they were included in the registry, are followed up to track the development of MS by linkage with an MS registry or a series of incident cases within a population including the cohort. The rate of disease in the …
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