December 14, 2012

Sackett's Levels of Evidence

1a: Systematic Reviews of Randomized Controlled Trials
A review of the literature that incorporates systematic methods to select studies in order to limit selection bias and provide reliable conclusions on a given topic. Only randomized controlled trials are included as part of the selection process.

1b: Randomized Controlled Trials
An experimental study design Subjects are randomly allocated to different groups. The groups are then compared in terms of the outcomes of interest. This design is chosen when testing the effectiveness of a treatment, or to compare several different forms of treatment.

2: Cohort Studies
An epidemiological research design that works forward from cause to effect, identifying groups of participants thought to have differing risks for developing a condition or characteristic and observing them over time to determine which group of participants are more likely to develop the condition or characteristic.

3: Case Control
An epidemiological research design in which groups of individuals with and without a certain condition or characteristic (the “effect”) are compared to determine whether they have been differentially exposed to presumed causes of the condition or characteristic

4: Case-Series
Systematic documentation of a well-defined unit; usually a description of an episode of care for an individual, but sometimes an administrative, educational, or other unit

5: Expert Opinion / Narrative Review
Narrative reviews tend to be mainly descriptive, do not involve a systematic search of the literature, and thereby often focus on a subset of studies in an area chosen based on availability or author selection. Thus narrative reviews while informative, can often include an element of selection bias.