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Evidence based practice – Systematic review, Randomized control trial, cohort study

Systematic Review

A systematic review is

a summary of the medical literature that uses explicit methods to perform a comprehensive literature search and critical appraisal of individual studies, and that uses appropriate statistical techniques to combine these valid studies.

Systematic reviews provide the strongest type of evidence, as the authors attempt to find all research on a topic, published and unpublished. The authors then combine the research into a single analysis.

Randomized Controlled Trials

A randomized controlled trial is an experimental, prospective study in which “participants are randomly allocated into an experimental group or a control group and followed over time for the variables/outcomes of interest.”

Study participants are randomly assigned to ensure that each participant has an equal chance of being assigned to an experimental or control group, thereby reducing potential bias. Outcomes of interest may be death (mortality), a specific disease state (morbidity), or even a numerical measurement such as blood chemistry level.

Frequently RCTs are used to measure the effectiveness of a particular therapy.

Cohort Studies

A cohort study is an observationalprospective or retrospective study. A cohort study “involves identification of two groups (cohorts) of patients, one that received the exposure of interest, and one that did not, and following these cohorts for the outcome of interest.”

While at first glance a cohort study looks similar to a RCT, it differs in one very significant way: the researchers do not assign the exposure or randomize the groups in any way. RCTs are experimental, while cohort studies are observational.

What researchers determine in a cohort study is the definition for group assignment.

Cohort studies may be prospective or retrospective. Retrospective studies “begin and end in the present but involve a major backward glance to collect information about events that occurred in the past.”

Prospective cohort studies can be extremely time-consuming. It may be necessary to follow a cohort for years or even decades to capture meaningful results. In that time study participants may be lost to follow up, potentially biasing the results. Retrospective cohort studies, on the other hand, are conducted on data that have already been collected, such as hospital records. This strategy saves time and expenses.

Case Control Studies

A case-control study is an observational, retrospective study which “involves identifying patients who have the outcome of interest (cases) and control patients without the same outcome, and looking back to see if they had the exposure of interest.”

Retrospective case-control studies rely on people’s memories, making them prone to error. Also, it may be difficult to measure the exact amount of an exposure in the past. Among people with bladder cancer, how might researchers determine the amount of artificial sweeteners used? Researchers might ask patients to self-report their estimated consumption. This method is inexact at best.

Case series

A case series is a descriptive report “on a series of patients with an outcome of interest. No control group is involved.”

Case series provide the weakest evidence of the study types examined so far, since they describe a relatively small number of patients and no experimental manipulation is involved. Case reports are simply descriptive reports of single patients. However, these study designs should not be ignored; case series and case reports often are used to introduce practitioners to unusual and rare conditions, or to point out “exceptions to the rule”. Furthermore, they are often the basis for future research using strong evidence study designs.

Editorials & expert opinion

The clinical experience, expertise, and judgment of a respected healthcare professional do play important roles in EBP. Sometimes there will not be methodologically sound research to answer your clinical question, and expert opinion will be important in your decision-making process. Keep in mind that both expert opinion and scientific research should be evaluated for selective use of evidence and other biases.