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When searching for evidence-based information, one should select the highest level of evidence possible--systematic reviews or meta-analyses. Systematic reviews, meta-analyses, and critically-appraised topics/articles have all gone through an evaluation process: they have been "filtered". 

Information that has not been critically appraised is considered "unfiltered".

As you move up the pyramid, however, fewer studies are available; it's important to recognize that high levels of evidence may not exist for your clinical question.  If this is the case, you'll need to move down the pyramid if your quest for resources at the top of the pyramid is unsuccessful.

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• Meta-Analysis  A systematic review that uses quantitative methods to summarize the results.

• Systematic Review    An article in which the authors have systematically searched for, appraised, and summarised all of the medical literature for a specific topic.

• Critically Appraised Topic     Authors of critically-appraised topics evaluate and synthesize multiple research studies.

• Critically Appraised Articles  Authors of critically-appraised individual articles evaluate and synopsize individual research studies.

• Randomized Controlled Trials  RCT's include a randomized group of patients in an experimental group and a control group. These groups are followed up for the variables/outcomes of interest.

• Cohort Study  Identifies two groups (cohorts) of patients, one which did receive the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.

• Case-Control Study  Involves identifying patients who have the outcome of interest (cases) and control patients without the same outcome, and looking to see if they had the exposure of interest.

• Background Information / Expert Opinion   Handbooks, encyclopedias, and textbooks often provide a good foundation or introduction and often include generalized information about a condition.  While background information presents a convenient summary, often it takes about three years for this type of literature to be published.

• Animal Research / Lab Studies  Information begins at the bottom of the pyramid: this is where ideas and laboratory
  research takes place. Ideas turn into therapies and diagnostic tools, which then are tested with lab models and
  animals.

Sources:
Greenhalgh, Trisha.  How to Read a Paper: the Basics of Evidence Based Medicine.  London: BMJ, 2000.
Glover, Jan; Izzo, David; Odato, Karen & Lei Wang. EBM Pyramid.  Dartmouth University/Yale University. 2006.

Different types of clinical questions are best answered by different types of research studies. 

You might not always find the highest level of evidence (i.e., systematic review or meta-analysis) to answer your question. When this happens, work your way down the Evidence Pyramid to the next highest level of evidence.

This table suggests study designs best suited to answer each type of clinical question.

Levels of evidence (sometimes called heirarchy of evidence) are assigned to studies based on the methodological quality of their design, validity, and applicability to patient care. These decisions gives the "grade (or strength) of recommendation".

The systematic review or meta-analysis of randomized controlled trials (RCTs) and evidence-based practice guidelines are considered to be the strongest level of evidence on which to guide practice decisions. (Melnyk, 2004) The weakest level of evidence is the opinion from authorities and/or reports of expert committees.

The following organizations describe levels of evidence:

AHRQ - Agency for Health Quality Research

Guide to Research Methods: The Evidence Pyramid  

Oxford Centre for Evidence-Based Medicine - Levels of Evidence (March 2009) 

Essential Evidence Plus: Levels of Evidence  

Center for Evidence Based Medicine

Consult these resources to understand the language of evidence-based practice and terms used in clinical research.