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Method of
overcoming bias
Randomisation
Blinding of participants
and investigators to
treatment allocation
Blind outcome assessment
Intention to treat analysis 
Definition
Systematic differences between comparison
groups in prognosis or responsiveness to
treatment
Systematic differences in care provided 
to participants in a study, apart from the
intervention being evaluated
Systematic differences in how outcomes 
are ascertained from the groups under
comparison
Systematic differences between comparison
groups in withdrawals or exclusions of
participants from the results of a study
Table 8.4
Potential biases affecting the internal validity of a study.
Bias
Selection bias
Performance bias
Measurement bias
Attrition bias
POOC08 02/18/2005 04:37PM Page 169

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Chapter 8
• The Users’ guide to the medical literature initially published in the
Journal of the American Medical Association and available online at
http://www.cche.net/usersguides/main.asp. This series has also
been updated and published as a book:
Guyatt G, Rennie D. Users’ Guide to the Medical Literature. A Manual
for Evidence-based Clinical Practice. Chicago, American Medical
Association, 2002.
• Crombie I. The Pocket Guide to Critical Appraisal: A Handbook for
Health Care Professionals. London, BMJ Publishing Group, 1996.
• The Appraisal of Guidelines Research and Evaluation (AGREE)
instrument provides a framework for evaluating the quality of 
clinical practice guidelines http://www.agreecollaboration.org/
Often a first attempt at critical appraisal can feel slow and cumber-
some, but with practice it becomes rapid and almost automatic. 
By developing skills associated with critical appraisal healthcare
workers will become more adept at managing the ever increasing
information overload. It is imperative that any healthcare worker
wishing to practise in an evidence based manner develops the ability
to quickly identify and interpret high quality, valid and clinically 
relevant research articles to inform their practice.