When you start researching you will encounter many different types of evidence such as systematic reviews, randomized controlled trails, 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.
Case Controlled studies are where researchers conduct a comparison of cases with a particular outcome and cases without a particular outcome to evaluate the participants' exposure.
Case Series/Case Report is a research design that track patients with a known exposure given similar treatment or examines their medical records for exposure and outcomes.
Cohort Studies with a control group are those where a group of people with something in common (a cohort) are followed. This group is compared to another group with similar characteristics/circumstances, with the exception of the factor being investigated.
Cross-Sectional studies involve data collected at a defined time, providing a snapshot of a disease in the population (observational studies).
Meta-analysis uses statistical methods to pool the results of independent studies (quantitative).
Meta-Synthesis is a qualitative analysis of a group of individual studies in which the finding of the studies are pooled.
Randomized Clinical Trails is an experiment using human beings in which the investigator randomly assigns participants in the trial either to a treatment or control (no treatment) group.
Sources
Hopp, L., & Rittenmeyer, L. (2012). Introduction to evidence-based practice: A practical guide for nursing. Philadelphia: F.A. Davis.
UIC Evidence Based Practice Tutorial, ebp.lib.uic.edu
Level A: Meta-analysis of multiple controlled studies or meta-synthesis of qualitative studies with results that consistently support a specific action, intervention or treatment.
Level B: Well designed controlled studies, both randomized and nonrandomized with results that consistently support a specific action, intervention, or treatment.
Level C: Qualitative studies, descriptive or correlational studies, integrative reviews, systematic reviews, or randomized controlled trials with inconsistent results.
Level D: Peer reviewed professional organization standards with clinical studies to support recommendations.
Level E: Theory based evidence from expert opinions or multiple case reports
Level M: Manufacturers' recommendations only
Sources
Armola RR, Bourgault AM, Halm MA, Board RM, Bucher L, Harrington L, Heafey CA, Lee R, Shellner PK, Medina J. (2009) AACN levels of evidence: what's new? J.Crit Care Nurse. Aug;29(4):70-3.
Peterson et. al. (2014). AACN Evidence Leveling System, Retrieved from https://pdfs.semanticscholar.org/67c4/545a68c7746af4a47b70d0e8f60a4e2fc25d.pdf