Generally, analytic research can be divided into categories of Observational and Experimental studies:
Observational Research: Observational studies do not allow for investigator allocation, sometimes due to ethical considerations. In these studies, the factors are self-selected, and so the empirical evidence is weaker due to possible confounding biases.
Experimental Research: In these studies, the investigator has control over allocation, and so randomization can be employed. For this reason, experimental studies tend to provide strong empirical evidence.
In addition, various filtered or secondary studies exist. They review and evaluate applicable studies, and sometimes perform additional statistical analysis on the cumulative data.