This guide will help you locate the highest levels of evidence: systematic reviews & meta-analyses, and practice guidelines.
If you can't locate the highest levels of research to answer a specific clinical question you should use the highest level available.
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.