Level of evidence refers to the rigor of the methods used to generate the evidence. Typically, the level that is assigned to evidence is based on where the study design sits on an evidence pyramid or hierarchy. For instance, evidence from a randomized control trial is typically considered to be at a higher level than evidence from a quasi-experimental study.
Quality of evidence refers to the execution or reporting of the study, in particular how the researchers minimized bias in their study.
A combination of level of evidence and quality of evidence can be used for an overall determination of the strength of evidence.
Critical appraisal is the process of carefully and systematically examining research to judge its trustworthiness, and its value and relevance in a particular context. Often checklists and rubrics are used to guide the critical appraisal process.
Determining the level of evidence for a particular study or information source depends on understanding, the nature of the research question that is being investigated and the methodology that was used to collect the evidence. See these these resources for help understanding study methodologies.
There are a number of evidence hierarchies that could be used to 'rank' evidence. Which hierarchy is applied often depends on disciplinary norms - students should refer to materials and guidance from their professors about which hierarchy is appropriate to use.
Understanding how a study was conducted (the methodology) is fundamental for determining the level of evidence that was generated by the study, as well as assessing the quality of the evidence it generated. While some papers state explicitly in the title what kind of method was used, it is often not so straightforward. When looking at report of a study, there are a few techniques you can use to help classify the study design.
In some bibliographic databases, there is information found in the Subject field, or the Publication Type field of the record that can provide information about a study's methodology. Try to locate the record for the article of interest in CINAHL, PubMed or PsycINFO and look for information describing the study (e.g., is it tagged as a "randomized controlled trial," a "case report," and "observational study", a "review" article, etc).
While there may be some information in the abstract that indicates a study's design, it is often necessary to read the full methods section in order to truly understand how the study was conducted. For help understanding the major types of research methodologies within the health sciences, see:
Once the study methodology is understood, a tool or checklist can be selected to appraise the quality of the evidence that was generated by that study.