Chapter 2What is

Evidence?

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.

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”

Preliminary Quality Criteria

• Relevance

– Question posed

– Patient involved

• Research process

– Peer review

– Contemporary timing

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.

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

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

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

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