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.
Level I - Evidence from a systematic review or meta-analysis of all relevant randomized controlled trials (RCTs)
Level II - Evidence obtained from well-designed RCTs
Level III - Evidence obtained from well-designed controlled trials without randomization
Level IV - Evidence from well-designed case-control and cohort studies
Level V - Evidence from systematic reviews of descriptive and qualitative studies
Level VI - Evidence from single descriptive or qualitative studies
Level VII - Evidence from the opinion of authorities and/or reports of expert committees
Q1. What was the aim of the study?
To simply describe a population (PO questions)
To quantify the relationship between factors (PICO questions)
Q2. If analytic, was the intervention randomly allocated?
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?
At the same time as the exposure or intervention?
Before the exposure was determined?
Source: Centre for Evidence Based Medicine: Study Designs. http://www.cebm.net/study-designs/