Primary literature (unfiltered)
Primary literature refers to content that is unique and original to the author(s). This type of research is unfiltered; other than the discussion of findings from the results, it has not undergone any additional meta-analysis or synthesis.
Examples of primary literature or unfiltered information include:
Secondary literature (filtered)
Secondary literature refers to content that summarizes primary literature. This type of research is filtered; the author(s) of secondary literature may or may not have been directly involved in the gathering of content for the primary literature, and the purpose of secondary literature is to synthesize, analyze, and review.
Examples of secondary literature or filtered information include:
The term "research" is often used to refer to primary literature. When a professor asks for students to include "research" in a health sciences-related assignment, this often means primary literature, such as originally-designed studies.
Tip: always check with your professor if you have any questions about your assignment's requirements.
The term "review" is often used to refer to a type of secondary literature. Reviews are not themselves original studies, but they will typically refer to one or more original studies that are then synthesized and analyzed in some way within the review article.
As you search on your topic, you may see many types of results, including those listed below. Please note: non-scholarly refers to resources that have not undergone a rigorous peer review process, which is mandatory for inclusion in scholarly journals. Non-scholarly resources are not inherently bad information; however, you may have restrictions on your assignments for the quantity and quality of non-scholarly resources you include.
In some journals, you will see a 'level of evidence' assigned to a research article. Levels of evidence are assigned to studies based on the methodological quality of their design, validity, and applicability to patient care. The combination of these attributes gives the level of evidence for a study. Many systems for assigning levels of evidence exist. A frequently used system in medicine is from the Oxford Center for Evidence-Based Medicine. In nursing, the system for assigning levels of evidence is often from Melnyk & Fineout-Overholt's 2011 book, Evidence-based Practice in Nursing and Healthcare: A Guide to Best Practice. The Levels of Evidence below are adapted from Melnyk & Fineout-Overholt's (2011) model.
Uses of Levels of Evidence: Levels of evidence from one or more studies provide the "grade (or strength) of recommendation" for a particular treatment, test, or practice. Levels of evidence are reported for studies published in some medical and nursing journals. Levels of Evidence are most visible in Practice Guidelines, where the level of evidence is used to indicate how strong a recommendation for a particular practice is. This allows health care professionals to quickly ascertain the weight or importance of the recommendation in any given guideline. In some cases, levels of evidence in guidelines are accompanied by a Strength of Recommendation.
About Levels of Evidence and the Hierarchy of Evidence: While Levels of Evidence correlate roughly with the hierarchy of evidence (discussed elsewhere on this page), levels of evidence don't always match the categories from the Hierarchy of Evidence, reflecting the fact that study design alone doesn't guarantee good evidence. For example, the systematic review or meta-analysis of randomized controlled trials (RCTs) are at the top of the evidence pyramid and are typically assigned the highest level of evidence, due to the fact that the study design reduces the probability of bias (Melnyk, 2011), whereas the weakest level of evidence is the opinion from authorities and/or reports of expert committees. However, a systematic review may report very weak evidence for a particular practice and therefore the level of evidence behind a recommendation may be lower than the position of the study type on the Pyramid/Hierarchy of Evidence.
About Levels of Evidence and Strength of Recommendation: The fact that a study is located lower on the Hierarchy of Evidence does not necessarily mean that the strength of recommendation made from that and other studies is low--if evidence is consistent across studies on a topic and/or very compelling, strong recommendations can be made from evidence found in studies with lower levels of evidence, and study types located at the bottom of the Hierarchy of Evidence. In other words, strong recommendations can be made from lower levels of evidence.
For example: a case series observed in 1961 in which two physicians who noted a high incidence (approximately 20%) of children born with birth defects to mothers taking thalidomide resulted in very strong recommendations against the prescription and eventually, manufacture and marketing of thalidomide. In other words, as a result of the case series, a strong recommendation was made from a study that was in one of the lowest positions on the hierarchy of evidence.
The pyramid below represents the hierarchy of evidence, which illustrates the strength of study types; the higher the study type on the pyramid, the more likely it is that the research is valid. The pyramid is meant to assist researchers in prioritizing studies they have located to answer a clinical or practice question.
For clinical questions, you should try to find articles with the highest quality of evidence. Systematic Reviews and Meta-Analyses are considered the highest quality of evidence for clinical decision-making and should be used above other study types, whenever available, provided the Systematic Review or Meta-Analysis is fairly recent.
As you move up the pyramid, fewer studies are available, because the study designs become increasingly more expensive for researchers to perform. It is important to recognize that high levels of evidence may not exist for your clinical question, due to both costs of the research and the type of question you have. If the highest levels of study design from the evidence pyramid are unavailable for your question, you'll need to move down the pyramid.
While the pyramid of evidence can be helpful, individual studies--no matter the study type--must be assessed to determine the validity.