• Case-control studies, and;

• Cohort studies.

For either of these two types of study, it is very important to specify inclusion and exclusion criteria.

The main difference in these two designs is that the Case-control study isretrospective in nature. The Cohort study is, in contrast, is prospective in nature.

The following diagram may help to understand the differences in these two types of design:

It is important to understand that I could have just as easily substituted "Treated" for "Exposed" and "Not treated" or "Alternative treatment" for "Non-exposed". Making this substitution would change "Diseased" to, e.g., "Cured" or "Improved" and "Non-diseased" or "No disease" to "Not cured" or "Not improved". If you are having trouble understanding my abstraction, click here.

Remember:

• Case-control works from outcome (or presence of disease) to treatment (or exposure).

• Cohort works from treatment (or exposure) to outcome (or presence of disease)