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Evidence-based Medicine: Appraise the Evidence

An interdisciplinary guide to EBM resources.

Step 3

Appraise: Critically appraise the literature: check for validity, clinical relevance, and applicability.

What to look for

"The three things to bear in mind are quality, validity, and size:

  • Quality: Trials that are randomised and double blind, to avoid selection and observer bias, and where we know what happened to most of the subjects in the trial.
  • Validity: Trials that mimic clinical practice, or could be used in clinical practice, and with outcomes that make sense. For instance, in chronic disorders we want long-term, not short-term trials. We are not interested in small but marginally statistically significant (p < 0.05, say, or a 1 in 20 chance of being wrong), but outcomes that are large, useful, and statistically very significant (p < 0.01, a 1 in 100 chance of being wrong).
  • Size: Trials (or collections of trials) that have large numbers of patients, to avoid being wrong because of the random play of chance. For instance, to be sure that a number needed to treat (NNT) of 2.5 is really between 2 and 3, we need results from about 500 patients. If that NNT is above 5, we need data from thousands of patients.

These are the criteria on which we should judge evidence. For it to be strong evidence, it has to fulfil the requirements of all three criteria."

Source:  Critical Appraisal. Bandolier.

Critically Appraised Topics (CATs)

CATs are critical summaries of a research article.  They are concise, standardized, and provide an appraisal of the research.

The University of Oxford's Centre for Evidence-Based Medicine provides a software program called CATMaker to download. This program guides users through the process to create one-page summaries of articles on therapy, diagnosis, prognosis, etiology/harm, and systematic reviews of therapy.