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Epidemiological evidence about the accuracy of diagnostic tests, the power of prognostic markers, and the efficacy and safety of interventions is the cornerstone of evidence-based health care.1 Practitioners of evidence-based health care require critical appraisal skills to judge the validity of this evidence. The Evidence-Based Medicine (EBM) Working Group members are international leaders in teaching critical appraisal skills, and their users’ guides for appraising the validity of the healthcare literature2 have long been the basis of teaching programmes worldwide. However, we found that many of our students took a reductionist “paint by numbers” approach when using the Working Group’s guides. Students could answer individual appraisal questions correctly but would have difficulty assessing overall study quality. We believe this is due to a poor understanding of epidemiological study design. So over the past 15 years of teaching critical appraisal, we have modified the EBM Working Group approach and developed the Graphic Appraisal Tool for Epidemiological studies (GATE) frame to help our students conceptualise the whole study as well as its component parts. GATE is a visual framework that illustrates the generic design of all epidemiological studies (figure 1⇓). We now teach critical appraisal by “hanging” studies and the EBM Working Group’s appraisal questions on the GATE frame.
This notebook outlines the GATE approach to critical appraisal, illustrated throughout using the Heart and Estrogen/progestin Replacement Study (HERS), a randomised, double blind, placebo controlled trial of the effect of daily oestrogen plus progestin on coronary heart disease (CHD) death in postmenopausal women.3 A detailed critical appraisal of HERS using a GATE-based checklist is available online.4
HANGING THE STUDY AND NUMBERS ON THE GATE FRAME
The GATE frame incorporates a triangle, circle, square, and arrow (figure 1⇑), labelled with the acronym PECOT (or PICOT).
The triangle (figure 2⇓) represents the population studied: “P” …
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