Why Search for Evidence-Based Literature
What does it mean to search for evidence-based literature? Isn't searching Google enough? The answer is No. Google, & other search engines, such as Bing or Yahoo, "scrape" information on the Internet using proprietary algorithms. This means that the search engines control your search, you don't. This may not be a problem if you're searching for things to buy, consumer reviews, or websites of organizations. But since search engines find information of all kinds, much of it opinion based, you need to use another tool when you're conducting evidence-based research. Article databases focus on the professional, scientific literature & are usually indexed, so that you can create a search to more precisely find reliable scientific evidence on your topic.
This guide will take you through the process of 1) creating a well-defined research question from a clincal scenario & 2) searching in article databases, such as PubMed, to find evidence-based literature.
Evidence Pyramid
The evidence pyramid shows you how reliable different types of research are. The lower levels contain information that is based on less evidence, such as expert opinion or very early experiments. Higher up the pyramid, the amount of literature decreases, but it is based on more evidence & thus is more reliable.
Types of Studies
Primary Literature (Unfiltered Literature)
Case series / Case reports: reports on the treatment of one or more individual patients. There are no control groups to compare outcomes. This type of research has little statistical validity.
Case control studies: patients who already have a specific condition are compared with people who do not have the condition, looking back in time to identify factors that might be associated with the illness. Case control studies often rely on medical records & patient recall for data collection, so their reliability is limited.
Cohort studies: compare a group of patients who are already under a specific treatment with a similar group not affected by the treatment. Both groups are are followed over time. Cohort studies are observational, so their reliability is limited.
Randomized controlled clinical trials (RCTs): carefully planned experiments that look at the effects of a treatment on real patients in real time. To reduce the potential for bias, randomization & blinding are used in the selection of patients & treatments. Patients are assigned to treatment & control groups randomly; patients & clinicians (& sometimes laboratories) do not know who is in which group & thus who is receiving treatment or not. This allows the groups to be directly compared. RCTs can provide sound evidence of cause & effect & are considered the gold standard for experimental research.
Secondary Literature (Filtered Literature)
Systematic Reviews: answer a specific question, using the results of an extensive literature search that identifies studies with sound & similar methodologies. The studies are reviewed, assessed for quality, & the results are summarized according to the standards of the question that is being reviewed.
Meta-analyses: thoroughly examine a number of valid studies on a topic & mathematically combine the results using statistical methodology to report the results of the analysis as if it were one large study.