In order to make medicine more evidence-based, it must be based on the evidence found in research studies with higher quality evidence having more of an impact than lower quality evidence. Evidence is ranked on a hierarchy according to the strength of the results of the clinical trial or research study. The strength of results can be impacted by a variety of factors such as the study design, outcomes, and bias, as well as the results themselves.

Q1. What was the aim of the study?

To simply describe a population (PO questions) 

• Descriptive

To quantify the relationship between factors (PICO questions) 

• Analytic

Q2. If analytic, was the intervention randomly allocated?

Yes? 

• RCT

No?

• Observational study

For observational study the main types will then depend on the timing of the measurement of outcome, so our third question is:

Q3. When were the outcomes determined?

Some time after the exposure or intervention?

• Cohort study (‘prospective study’)

At the same time as the exposure or intervention?

• Cross sectional study or survey

Before the exposure was determined?

• Case-control study (‘retrospective study’ based on recall of the exposure)

Source: Centre for Evidence Based Medicine: Study Designs. https://www.cebm.ox.ac.uk/resources/ebm-tools/study-designs