Observational studies. We observe one or more groups of subjects and their characteristics are described and/or analyzed.

Experimental. We can manipulate variables.

Series of cases. To describe characteristics of a patients group. Leads to the generation of hypothesis; use a short time period and there is not a control group.

Cases and controls. The beginning is with the presence or absence of outcome y review before in the time to detect risk factors.

Cohort. The beginning is with a subjects group with outcome; then, we classified them in exposed or non-exposed, and follow in the time to see if they develop the outcome.

Cross-sectional. We measure the exposure and the outcome in a subjects group simultaneously, in a point of the time.

Experimental. The beginning is with a cohort and randomize it to receive the intervention (experimental group) or not receive it (control group); then we follow to the subjects in the time to see if develop the outcome of interest.

Ecologic. The main characteristic is that study groups not individuals.