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Chapter 2What is
Evidence?
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Objectives
• Discuss the concept of “best available clinical evidence.”• Describe the general content and procedural
characteristics of desirable evidence and their implications for the selection of studies to evaluate.
• Describe different forms of evidence and their uses for answering clinical questions in physical therapist practice.
• Discuss and apply the principles and purposes of evidence hierarchies for each type of clinical question.
• Discuss the limitations of evidence hierarchies and their implications for the use of evidence in practice.
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What Qualifies as Evidence?
• Guyatt and Rennie - “any empirical observation about the apparent relation between events constitutes potential evidence.” For example:– Published research articles– Clinical practice guidelines– Patient/client records– Clinician recall of prior patient/client cases
• Sackett et al – “best available clinical evidence”
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Preliminary Quality Criteria
• Relevance
– Question posed
– Patient involved
• Research process
– Peer review
– Contemporary timing
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Research Design Overview• Design options that address bias
– Randomization techniques to distribute subjects into groups;
– The use of more than one group in order to make a comparison;
– Controlled experimental manipulation of the subjects;– Measures at the patient/client level (e.g., impairment in
body functions and structure, activity limitations, participation restrictions); and/or
– A systematic method for collecting and analyzing information.
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Research Designs - Timing• Prospective – a research design that follows subjects forward
over a specified period of time.
• Retrospective – a research design that uses historical (past) data from sources such as medical records, insurance claims, or outcomes databases.
• Cross-sectional – a research design that collects data about a phenomenon during a single point in time or once within a defined time interval.
• Longitudinal – a research design that looks at a phenomenon occurring over time
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Research Design – What is the Question?
• The question posed for study guides choices about the research design
• For example:
– Usefulness of diagnostic test/clinical measure – researchers may choose a cross-sectional non-experimental design
– Effectiveness of intervention – researchers may choose a cross-sectional or longitudinal experimental design
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Evidence Hierarchies
• Ranking schemes in which research designs are ordered from highest to lowest in terms of bias control
• Intended to facilitate the efficiency of the evidence selection process
• Details vary because research designs are dictated by the question posed
• Systematic reviews of high quality studies are at the top of all traditional hierarchies
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Evidence Hierarchy Limitations
• Variability in quality rating systems used to classify evidence
• Inconsistent reliability and validity testing
• Serve as a “screening tool” only – study or pre-appraised product must be evaluated on its own merits