Learn what types of evidence to look for.
Levels of Evidence
When searching for information, you want to select articles or studies with the highest evidence level possible. If there are no studies available for the best evidence level, move down to the next appropriate level and search for evidence there.
Based on the type of clinical question you are trying to answer, the type of evidence you will need to look for changes.
Understanding Study Types
Background Information and Expert opinions
Background knowledge and expert opinions represent the lowest level of evidence for informing evidence-based practice. This type of evidence may take the form of a book chapter, an editorial in a journal, or an entry in an expert-based resource like Up-To-Date. Because these resources are not the result of high-quality research, they are useful for learning more about a topic but not for applying to your practice as the evidence base.
Here are some examples of background knowledge and expert opinions:
Case Reports and case studies
Case studies and case reports represent one of lower levels of evidence in the medical literature. These are typically detailed descriptions of an unusual manifestation of an illness or condition in a particular patient, including the symptoms, diagnosis and treatment.
Here are some examples of case studies and case reports:
Case-control studies are a higher level of evidence than both case reports and expert opinions. They compare patients that have a disease or outcome to patients that do not have a disease or outcome by looking back through these patients histories to compare how frequently exposure to a risk factor of interest occurred in each group.
Here are some examples of case-control studies:
Cohort studies are often the highest level of evidence that can be found for both etiology and prognosis questions. In cohort studies, a large group of subjects without a disease are defined by a variable (like birth year, etc.). At set periods of time in the future, they are studied to see if they have developed a disease or condition of interest, and this helps to define a causal link between a risk factor and a disease. Unlike case-control studies which look backward in time after someone already has a disease, cohort studies follow a group of people forward in time to see if the disease develops.
Here are some examples of cohort studies:
Randomized Controlled Trials
Randomized controlled trials (RCTs) are one of the highest levels of evidence because they can statistically prove that an intervention has an effect on an outcome. RCTs are studies that involve the random allocation of participants to two groups: one group that receives an intervention or treatment, and one group that receives standard care or a placebo. The clinician conducting the study is blinded to which participants will be assigned throughout the trial, so results are unbiased.
Here are some examples of randomized controlled trials:
Forming the second highest level of evidence, systematic reviews are a comprehensive review of the existing medical literature meeting a set of eligibility criteria as it pertains to a pre-defined research question. What this means is that researchers create a systematic, reproducible search strategy to uncover all related articles. They then analyze all of the articles (usually randomized controlled studies) related to a clinical question and that meet the criteria for inclusion, and summarize the findings. The researchers then make recommendations for clinical practice based on the strength of the evidence they find.
Here are some examples of systematic reviews:
A meta-analysis is a type of systematic review that combines through statistical analysis the data from multiple studies, and identifies a common treatment effect. This is considered the highest level of medical evidence, and is thus placed at the top of the pyramid.
Here are some examples of meta-analyses: