The second research design with higher quality of information, only after the clinical trial is the cohort, although it does not have the possibility of assigning the maneuver, it has the opportunity to follow subjects over time. Any research that tries to explain the phenomenon of causality runs the risk of bias, however, the characteristics of the cohort studies try to avoid them. Its main features are: 1. Be observational, where the researcher only measures the presence of the maneuver, characteristic that divides subjects into exposed and unexposed. 2. Be longitudinal, which provides the opportunity to track the subject through time documenting the temporal sequence of components ocurrence. 3. The directionality in measurements, generating what we know as prolective, retrolective and retro-prolective cohort studies; the former is the one with the highest quality as a result of the measurement of the variables in real time. 4. Be a comparative study.