The hierarchy of evidence (or evidence hierarchy) is: "A ranked arrangement of the strength of research evidence based on the rigor of the method that produced it..." (Polit & Beck, 2022, 384).
In plain terms? While all evidence is good, there is some evidence that provides better data or stronger correlations for practice than others. For instance, a case study can provide good evidence that an intervention works, but when compared against a randomized controlled trial, where multiple patients and centers trial the same intervention, the data from the randomized controlled trial holds greater weight than the case study, as more participants participated in the study and there is more evidence.
Essentials of Nursing Research by A rigourous synthesis of research findings on a particular research question, using systematic sampling, data collection, and data analysis procedures specified in a formal protocol. (Polit & Beck, 2022, p. 401)
Systematic reviews typically take any number of individual studies and use the data to run a statistical analysis. When systematic reviews use randomized controlled trials and their data, this is usually considered the highest level of evidence, as you have several studies that support a research question.
A technique for quantitatively integrating the results of multiple studies addressing the same or highly similar research questions (Polit & Beck, 2022, p. 392).
Essentially, this means that researchers have taken the data from multiple quantitative studies that are on the same topic and conclude that the intervention or implementation works. Systematic reviews and meta-analyses of quantitative studies and randomized controlled trials are considered to be the highest level of evidence due to the fact that they take large amounts of quantitative data and analyze it to conclude that something works or establish a trend.
Systematic reviews of qualitative data are not held at the same level of systematic reviews of quantitative data due to the fact that qualitative data can be difficult to replicate or can be inconclusive.
A full experimental test of an intervention, involving random assignment of participants to different treatment groups (Polit & Beck, 2022, p. 397).
When researchers are looking into an intervention, they will have a variable group that receives the intervention and they will have a control group that does not receive the intervention. Participants are assigned to each group at random and the majority of the time, they do not know if they are in the control group or in the intervention group. This is to ensure that there is no bias in the trial among the participants.
A clinical trial in which two sets of people are blinded with respect to the group that a study participant is in; often a situation in which neither the participants nor those administering the treatment know who is in the intervention or control group (Polit & Beck, 2022, 383).
A double-blind study means that neither the participants in the study nor those administering the intervention know who is in the control group and who is in the variable group. This is a type of randomized controlled trial that contains another layer of randomness to better ensure there is no bias whatsoever in the trial.
A method involving a thorough, in-depth analysis of an individual, group or other social unit (Polit & Beck, 2022, p. 377).
A nonexperimental design that compares 'cases' (people with a specified condition) to matched controls (similar people without condition), to examine differences that could have contributed to 'caseness' (Polit & Beck, 2022, p. 377).
A nonexperimental design in which a defined group of people (a cohort) is followed over time to study outcomes for subsets of the cohorts (Polit & Beck, 2022, p. 378).
Case studies, and those with case-control and cohort designs, observe trends and instances of an intervention working and report on it. Case studies generally are not as high on the hierarchy of evidence, as they do not contain any sort of experiment and are contained to a small group of people, or report on one case, whereas larger studies will involve large groups of people and have some sort of control and/ or variable.
A summary of research on a topic, often prepared to put a research problem in context or to summarize existing evidence; typically, less rigorously conducted than a systematic review (Polit & Beck, 2022, p. 389).
Literature reviews, which can be great articles to get information from, should not be considered original research, as there was no study performed. Rather, the researchers looked for pre-existing data from other studies and summarized that information in order to come to a conclusion that a particular method or intervention works. Literature reviews sit lower on the hierarchy of evidence as the researchers did not create a study and collect data- they instead relied on the data from other studies.
You may be thinking then how does a literature review separate itself from a systematic review? Systematic reviews usually involve taking large data sets and using a meta-analysis to run pre-existing data through a new algorithm or statistical analysis to come up with new data or to draw a conclusion. Literature reviews usually select certain studies to pull information from and the number of studies is usually less than a systematic review. As the researchers are not conducting their own study, literature reviews are not considered original research.
Singh, A. P. (n. d.). What is hierarchy of evidence? Bone and Spine. https://boneandspine.com/what-is-hierarchy-of-evidence/