The levels of evidence pyramid provides a way to visualize both the quality of evidence and the amount of evidence available. For example, systematic reviews are at the top of the pyramid, meaning they are both the highest level of evidence and the least common. As you go down the pyramid, the amount of evidence will increase as the quality of the evidence decreases.
1, 2, 3 : Filtered resources (the quality of studies are appraised and often make recommendations for practice e.g. guidelines)
4, 5, 6 : Unfiltered resources (you review what you find to make sure it is valid and reliable)
7 : Background information, expert opinion (not necessarily backed by research studies)
Systematic reviews can often provide the best evidence. They research findings in thorough systematic literature searches. But:
If a current systematic review isn't available, go to primary studies e.g. papers directly from the researcher who conducted the study. The table below lists optimal study methodologies for the main types of questions.
|Question||Best Research Design for Question|
|Therapy (Treatment)||Randomised Controlled Trial (RCT)|
|Prevention||RCT or Prospective Study|
|Diagnosis||RCT or Cohort Study|
|Prognosis (Forecast)||Cohort Study and/or Case-Control Series|
|Etiology (Causation)||Cohort Study|
Note that the Clinical Queries filter available in CINAHL/Medline matches the question type to studies with appropriate research designs.
When searching primary literature, look first for reports of clinical trials that used the best research designs. Remember as you search, though, that the best available evidence may not come from the optimal study type. For example, if treatment effects found in well designed cohort studies are sufficiently large and consistent, those cohort studies may provide more convincing evidence than the findings of a weaker RCT.