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.
- Hierarchy of Evidence
- Types of Questions & Study Types
- Study Designs
Scientific evidence is not created equal. The hierarchy of evidence is represented by the evidence pyramid. The study with the highest level of evidence, i.e., systematic review or meta-analysis is at the top of the pyramid, followed by randomized controlled trials [RCTs], observational studies and the lowest level evidence being case reports, case-series, or expert opinions residing at the bottom of the pyramid.
Greenhalgh T. [2010]. How to read a paper: the basics of evidence-based medicine [4th ed.]. Wiley-Blackwell.
The following table shows an example of the study design that best answered the different types of commonly seen clinical questions.
Fineout-Overholt, E., & Johnston, L. [2005]. Teaching EBP: asking searchable, answerable clinical questions. Worldviews Evid Based Nurs, 2[3], 157-160. //doi.org/10.1111/j.1741-6787.2005.00032.x
"The choice of study type will mainly depend on the research question being asked." The article on "What types of studies are there?" gives a brief description to the different study designs.
Adapted from InformedHealth.org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care [IQWiG]; 2006-. What types of studies are there? 2016 Jun 15 [Updated 2016 Sep 8]. Available from: //www.ncbi.nlm.nih.gov/books/NBK390304/
Randomized controlled trials [RCTs] | RCTs provide the best results when trying to find out if there is a cause-and-effect relationship. RCTs provide the most reliable answers for treatment/therapy/intervention/diagnostic test where they can how what happens if you opt to not have the treatment or diagnostic test or compared to other treatments or diagnostic tests. In order to be able to reliably assess how effective the treatment is, the following things also need to be determined before the study is started:
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Cohort studies | Best to answer prognosis, harm and causation type of questions as a group of people who are observed frequently over a period of many years. As it might NOT be ethical to conduct a RCT to find out if smoking causes cancer. Therefore, using cohort studies, the researchers then observe how the health of the people in both groups [smoke and non-smoke] develops over the course of several years, whether they become ill, and how many of them pass away. Cohort studies can have a prospective [forward-looking] design or a retrospective [backward-looking] design. Cohort studies are especially useful if you want to find out how common a medical condition is and which factors increase the risk of developing it. They can answer questions such as:
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Case-control studies | Second best to answer harm/causation/prognosis type of questions as it is not as expensive or time-consuming as RCTs or cohort studies. But it is often difficult to tell which people are the most similar to each other and should therefore be compared with each other. Usually data gathered based on past events which are dependent on the participants’ memories. Still, case-control studies can help to investigate the causes of a specific disease, and answer questions like these:
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Cross-sectional studies | Use to answer screening/diagnosis type of question. The classic type of cross-sectional study is the survey: A representative group of people – usually a random sample – are interviewed or examined in order to find out their opinions or facts. Unlike cohort studies, cross-sectional studies can be relatively quick and inexpensive as data is collected only once. Cross-sectional studies can answer questions such as these:
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Qualitative Studies | Unlike other kinds of research, qualitative research does not rely on numbers and data. Instead, it is based on information collected by talking to people who have a particular medical condition and people close to them. Make use of a number of methods such as interviews, diaries, observation, focus groups [both textual and visual analysis] to interpret the findings. Qualitative studies can answer questions such as these:
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