EVIDENCE BASED MEDICINE: RESEARCH DESIGNING

Evidence based medicine is the conscientious, explicit, and judicious use of current best evidence in making decisions about the care of individual patients. The practice of evidence based medicine means integrating individual clinical expertise with the best available external clinical evidence from systematic research.

The ability to incorporate Evidence-Based Medicine into clinical care requires a basic understanding of the main research designs underlying the published evidence. Some research designs provide a stronger level of evidence than others based on their inherent characteristics. This hierarchy is often shown graphically as a pyramid:

Levels of Evidence

The pyramid represents the quality of research designs by level, as well as the quantity of each study design in the body of published literature. Systematic reviews (higher quality), for instance, are the most time-intensive articles to write and are therefore rarer (lower quantity) than other types of studies.

More detailed levels of evidence have been developed by the Oxford Centre for Evidence-Based Medicine. They use a numbering scheme ranging from 1a, homogenous systematic reviews of randomized controlled trials, to 5, expert opinion. This system can be especially useful when comparing articles with similar study designs. Equivalent research designs do not always produce results of equal quality.

Though finding research studies high on the pyramid is preferred, Evidence-Based Practice may need to draw on research designs lower in the evidence hierarchy than case series. Occasionally, nothing but case reports or even bench research may exist on a topic. When making evidence-based decisions for patient care, it is essential to select the highest level research design available for the specific question of interest.
Systematic reviews
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reviews provide the strongest type of evidence, as the authors attempt to find all research on a topic, published and unpublished.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. Keep in mind that systematic reviews are different than review articles. While systematic reviews are conducted to answer a specific clinical foreground question, review articles provide a broad overview on a topic to answer background questions. Another difference is that the literature search for review articles does not attempt to find all existing knowledge on a topic.
Meta-Analysis
A meta-analysis is a particular type of systematic review that attempts to combine and summarize quantitative data from multiple studies using sophisticated statistical methodology. Such a strategy strengthens evidence as it makes the small sample size of individual studies much larger, giving the results more statistical power and, therefore, more credibility than the individual studies. Meta-analyses are not comprehensive, as only compatible data may be combined into a larger data set.
Authors should clearly specify the criteria for inclusion or exclusion of individual studies somewhere in a systematic review or meta-analysis.
Randomized controlled trials (RCT)
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.
A typical RCT that represents the flow of participants from the start of the study through the study outcome. Notice in all diagrams the study start; studies progressing from left to right represent prospective studies, “collecting data about a population whose outcome lies in the future”.

Frequently RCTs are used to measure the effectiveness of a particular therapy, especially drug therapy.
Cohort studies
A cohort study is an observational, prospective 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 forward 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.

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.”

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. 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.

by

Akshaya Srikanth,

Pharm.D Internee,

FIP-YPG Associate,

Hyderabad, India