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.
It is important to distinguish the level of evidence from critical appraisal of the quality of a study. Determining a study's position on the evidence hierarchy is driven by the study's design. Critical appraisal is a systematic process of evaluating the strengths and weaknesses of an individual study in order to make a judgement about the validity of its findings, as well as its relevance and value within a particular context.
It is possible to have a study that sits relatively high on the evidence pyramid based on its methodology (e.g., a randomized controlled trial) that you ultimately judge to be of low quality, if the design of the study failed to rigorously account for biases. There are a number of tools and checklists available to guide you through the process of critical appraisal; these tools help you think through the common sources of bias and threats to internal validity in different study designs, allowing you to make an overall evaluation of the quality of a study.
The list below gives links to some of critical appraisal tools and reporting standards that are available for particular study designs and resource types. For a more complete list, see the guide to Critical Appraisal Resources.