P patient problem, population, or situation
I intervention, including an exposure, a diagnostic test, a prognostic factor, a treatment, a patient perception, and so forth.
C comparison intervention or exposure (also defined very broadly), if relevant.
O outcome(s) of interest, including a time horizon, if relevant.
Synthesized resources are a specialized form of secondary literature and focus on critical appraisal of a number of primary resources. They most commonly address clinical topics and appear as systematic reviews, meta-analyses, clinical practice guidelines, critically appraised topics, or decision analysis.
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Levels of Evidence (LOE) correlate to study/publication types. The pyramid represents LOEs and the sheer size of the related literature.
Authors of a systematic review ask a specific clinical question, perform a comprehensive literature search, eliminate the poorly done studies and attempt to make practice recommendations based on the well-done studies. A meta-analysis is a systematic review that combines all the results of all the studies into a single statistical analysis of results.
Systematic reviews and meta-analyses are the highest level of evidence followed by the randomized controlled trials. There are only 8000 systematic reviews in the Cochrane database compared to the over 22 million citations indexed in Medline.
Studies are experimental or observational, prospective or retrospective.
See the University of Illinois Research Design - Study Type website for clear descriptions and examples.
Different categories of questions are best answered by different research designs.
Therapy RCT preferred, otherwise: cohort study, case-control study, case series.
Diagnosis Prospective, blind comparison to a reference standard.
Etiology or Harm RCT preferred otherwise: cohort study, case-control study, case series.
Prognosis Cohort study preferred otherwise: case-control study, case series.
Phase I trials test an experimental drug or treatment in a small group of people (20-80) for the first time to evaluate its safety, determine a safe dosage range, and identify side effects.
Phase II trials test a larger group of people (100-300) to see if it is effective and to further evaluate its safety.
Phase III trials the experimental study drug or treatment is given to large groups of people (1,000-3,000) to confirm its effectiveness, monitor side effects, compare it to commonly used treatments, and collect information that will allow the experimental drug or treatment to be used safely.
Phase IV trials are post marketing studies to delineate additional information including the drug's risks, benefits, and optimal use.