When you start researching you will encounter many different types of evidence such as systematic reviews, randomized controlled trials, clinical guidelines and opinion articles. These different resources will not all have the same "weight" in terms of reliability and trustworthiness. To assist you in determining what is the most reliable, the levels of evidence hierarchies will guide you.
Evidence Hierarchies are systems used to rank evidence according to certain criteria. There are many hierarchies, including the examples on this page.
Hopp, L., & Rittenmeyer, L. (2012). Introduction to evidence-based practice: A practical guide for nursing. Philadelphia: F.A. Davis.
These resources provide additional information on the levels of evidence.
Melnyk, B.M. & Fineout-Overholt, E. (2010). Evidence-based practice in nursing & healthcare: A guide to best practice (2nd edition). Philadelphia, PA: Wolters Kluwer/Lippincott Williams & Wilkins.
The Joanna Briggs Institute adopted a new hierarchy for levels of evidence as of March 1, 2014. The chart below outlines the levels of evidence for effectiveness questions.
The Joanna Briggs website contains levels of evidence charts for other types of questions.
This evidence pyramid provides a concept of higher to lower levels of evidence.
Source: UIC Evidence Based Practice Tutorial, ebp.lib.uic.edu