Not all evidence is judged to be of equal value; that is, there are hierarchies of research design that are evaluated to have different strengths and different levels of value in the decision making process.  See the charts below – one graphically represented, one textually - to help understand the concepts important to critical appraisal, assessment, and evaluation of research.

[Source: HealthLinks, University of Washington: http://healthlinks.washington.edu/ebp/ebptools.html

Levels of Evidence

• Category I:  Evidence from at least one properly randomized controlled trial.
• Category II-1: Evidence from well-designed controlled trials without randomization.
• Category II-2: Evidence from well-designed cohort or case-control analytic studies, preferably from more than one center or research group.
• Category II-3: Evidence from multiple times series with or without intervention or dramatic results in uncontrolled experiments such as the results of the introduction of penicillin treatment in the 1940s.
• Category III: Opinions of respected authorities, based on clinical experience, descriptive studies and case reports, or reports of expert committees (Clinical wisdom).

[Source: Harris, R.P. et al. (2001). Current methods of the U.S. Preventive Services Task Force: a review of the process. American Journal of Preventive Medicine. April 20 (3 Supplement): 21-35.]

Practice Guidelines

Professional associations such as APA have set up their own criteria for deciding if a practice intervention is evidenced-based.

In Psychology, "The term guidelines refers to pronouncements, statements, or declarations that suggest or recommend specific professional behavior, endeavors, or conduct for psychologists (American Psychological Association [APA], 1992. Guidelines differ from standards in that standards are mandatory and may be accompanied by enforcement mechanism (APA, 2001)...federal or state law may supercede these guidelines."

[Source: APA (2003). Guidelines on multicultural education, training, research, practice, and organizational change for psychologists. American Psychologist, May 2003, pg. 378.]

Example of Practice Guidelines: APA Practice Guidelines http://www.psych.org/psych_pract/treatg/pg/prac_guide.cfm

The social work profession does not have one agreed upon criteria for evidence-based practice. For example, NASW policy statements in Social Work Speaks do not cover evidence-based practice. However, a new resource from the Social Work Leadership Institute: Center on Aging Policy provides an Evidence Database: http://socialworkleadership.org/nsw/cap/ebp.php

"The Evidence Database is regularly updated by an advisory panel that filters, reviews, and catalogues articles published in professional journals both in the U.S. and abroad. The database allows you to refine your search through multiple keywords and categories.

Federal agencies such as SAMHSA set up their own guidelines.

The National Registry of Evidence-based Programs and Practices (NREPP) is a searchable online registry of mental health and substance abuse interventions that have been reviewed and rated by independent reviewers at http://www.nrepp.samhsa.gov.