Professional bodies assess the quality of relevant studies when developing standards and guidelines. ACOG, for example, uses the following methodology (USPSTF 2001, see ‘Learn More-Primary Sources’) and one can find the specific level of evidence after each reference.
Using the highest level of evidence following data review, ACOG will provided graded recommendations
Level A Recommendations are based on good and consistent scientific evidence
Level B Recommendations are based on limited or inconsistent scientific evidence
Level C Recommendations are based primarily on consensus and expert opinion
The concept of ‘levels of evidence’ follows the NAM standard that quality of evidence should be an integral part of medical guideline development. Note that the NAM does not prescribe a single rating system. Different professional bodies may use different scoring systems. What is important is that ‘levels of evidence’ be readily apparent to the guideline reader. The same approach can be used when reviewing a paper.
Assess the quality of a study by asking 3 basic questions:
(a) Cohort study – these are studies where the starting point is a particular exposure
(b) Case control studies – these are ‘retrospective’ studies – which means that the events or interventions being studied have already taken place in the past and the outcome of interest drives the design, rather than the exposure (see cohort study, above)
Cross sectional studies – At any given time, what is the exposure and outcome?
“Level of evidence” is a standardized way to determine the quality of a research project, which is based on study design. Clinical trials are ‘interventional’, if researchers intervene and are the highest level of evidence, followed by lower levels of evidence where observational approaches are used and ‘nature is allowed to take its course’. Ultimately, the goal of any such rating system is to help clinicians provide evidenced-based care to patients and aid in public health policy.
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