Critically Evaluate the Evidence
The previous steps describe how to formulate an answerable clinical question and how to search for evidence using computer databases. This section outlines how to evaluate the quality of the articles that were found. Not all studies are equally strong in terms of determining cause and effect. Unless one has been fortunate enough to find a systematic review or position statement about his or her question, one will have to form an opinion about the best clinical practice. To do
Taping that, one will need to weigh the strength of the evidence available. There are 5 key aspects to this evaluation, including the following:
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1. Identify the research study design and know its strengths and weaknesses.
2. Consider the similarities of the study participants to the patient. (Is the entire range of patients included? Is the patient included in the range?)
3. How were treatments conducted? (Determine the quality of the methods used.)
4. What was the accuracy of the outcome measurements? (Were outcomes compared to a criterion? Were clinically useful statistics used?)
5. What other explanations exist for the results? (What limitations may have affected these results?)
Research Study Design and Clinical Questions
The hierarchy of study designs was introduced in Post 3 (see Figure 5-1). Recall that Level I evidence comes from meta-analyses and position statements from medical organizations. As the levels progress downward, the impact of the research decreases, as does the value of the evidence. In the absence of systematic reviews, a thorough critical appraisal of the studies is needed. Table 5-3 summarizes the type of study and which clinical question can be answered by that design. It is important to understand the basic types of research designs that can be undertaken. They generally fall into 3 categories (descriptive, correlational, and experimental [cause and effect]). Each has its benefits and limitations. The 3 types can be viewed as a continuum from simple to complex. Descriptive questions are considered the simplest, and experimental studies are the most complex. A correlation study assumes that one can first describe (by measuring or observing) each variable. An experimental study assumes that the variables are related to each other. Experimental designs are the best design to determine causal relationships. A summary of the purpose, characteristics, limitations, and statistical analysis of each type are summarized in Table 5-4. Once a database search has been completed, one may have several articles to choose from to an attempt to answer a clinical question. There will be several types of studies, so it is important to choose article(s) that use a research design that is best suited to the clinical question one wants to answer and also ones that provide the best evidence. If one has several articles to choose from in answering a clinical question, choose the articles that are as close to the top of the pyramid as possible. The Oxford Centre for Evidence-Based Medicine (CEBM)4 created a table matching the levels with the type of clinical question one may be asking. It can be downloaded from www.cebm.net/ocebm-levels-of-evidence/. A simplified version is found in Table 5-4.
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