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Chapter: Medicine Study Notes : Evidence Based Medicine

Evidence Based Medicine

What is Evidence Based Medicine: „The conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients. This practice means integrating individual clinical experience with the best available external clinical evidence from systematic research‟

Evidence Based Medicine



What is Evidence Based Medicine


·        = „The conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients. This practice means integrating individual clinical experience with the best available external clinical evidence from systematic research‟ 

·        Why is it necessary:

o   Wide variation in clinical practice

o   To minimise clinical error

o   High use of treatments not proven to do more good than harm

·        Can be extended from medicine to health care 

·        Need to understand new types of evidence: randomised controlled trials (started in 1948), meta-analysis and systematic reviews 

·        EBM approach is better than traditional continuing medical education (which is largely ineffective). Clinical knowledge and performance deteriorates over time

·        Application of evidence:

o   Generalisability: do trials apply to a whole population

o   Applying probabilities from a population to an individual

o   Multiple treatments: trials only test one at a time

o   Acceptability

o   Also need to consider economic impact given limited resources


Developing a CAT (Critically Appraised Topic)


·        Title: Answer to the clinical question that initiated the search

·        Clinical Scenario: brief summary of the context in which the question arose 

·        The clinical bottom line: a description of the clinical action to be taken as a result of the critical appraisal

·        Well Built Question: good for defining pre-test probability and narrowing search strategy 

o   The Patient or problem being addressed in what setting (e.g. In otherwise healthy, non-smoking adults presenting to a GP with a common cold…)

o   The intervention being considered (…does a 5 day course of XX ….)

o   The comparison intervention (where relevant) (…compared with a placebo…) 

o   The clinical outcome of interest (…improve symptomatic control and/or reduce the duration of an existing common cold)

·        Search The Evidence: list search terms

·        Description of study methodology

·        Table summarising key results

·        Quantitative take home messages 

·        Notes on validity issues: bias, levels of evidence, research design and methodology, harmful effects of intervention, etc

·        References to the articles used in the critical appraisal


Cochrane Collaboration


·        Aims:

o   Evidence based

o   Easily accessed

o   Clinically useful

o   Quality controlled

o   Periodically updates

·        Prepares, maintains and disseminates systematic reviews.

·        Systematic reviews are a structured process:

o   Well formed question

o   Comprehensive data search (including non-English, unpublished)

o   Unbiased selection and abstraction process

o   Critical appraisal of data

o   Synthesis of data

o   Width of diamond on Cochrane logo = confidence interval of meta-analysis


Levels of Evidence


·        1 – RCT or reviews of them

·        2 – Cohort studies or reviews of them

·        3 – Case control studies

·        4 – Case series or poor quality cohort or case-control studies

·        5 – Expert Opinion


EBM Glossary


·        Bias: systematic deviation of study results from true results due to the study design.

o  Interviewer bias: systematic error due to interviewer‟s gathering of selective data. 

o  Lead time bias: if patients not enrolled at similar point in their illness, differences in outcome may only reflect differences in duration in illness. 

o  Recall bias: systematic error due to differences in accuracy or completeness of recall. Referral filter bias – process of referral from primary to secondary ­ proportion of severe cases ® ­unfavourable outcomes. 

o  Selection bias: a bias in study design rather than chance when study and control groups differ in ways that may affect the outcome

o   Publication Bias: results from studies with positive results are more likely to be published

·        Study types: 

o  Case-Control Study: retrospective comparison of exposures of persons with disease and without disease 

o  Prospective Study: cohorts who have not yet had the outcome event are monitored for the occurrence of the event 

o  Systematic Review: study in which trials on a topic have been systematically identified, appraised and summarised according to predetermined criteria. May or may not include a meta-analysis combining the results of the trials


·        Clinical Practice Guideline: systematically developed statement to assist decision-making in specific clinical circumstances

·        Study Design: 

o  Decision Analysis: application of explicit, quantitative methods to analyse decisions under conditions of uncertainty 

o  Intention to treat analysis: analyses individuals according to the group to which they were randomised, even if they didn‟t receive the treatment, rather than confining to those who completed treatment (treatment may have intolerable side-effects). Better measure of effectiveness. 

o  N-of-1 trials: blinded patient and doctor undergoes pairs of treatment periods with experimental and placebo treatment with outcomes monitored

·        Study Analysis: 

o  Confounding: a variable associated with the factor under investigation. Unless it is possible to adjust for the confounding variables, their effects cannot be distinguished from those of the factors being studied 

o  Efficacy: benefit of an intervention under ideal conditions

o  Efficiency: benefit of intervention, including efficacy and acceptance (e.g. compliance, side effects        does it do more harm than good)

o  Precision: the range in which the best estimates of a true value approximate the true value

o  Statistical power: statistical chance of a study being able to detect a difference if one actually exists 

o  Strength of inference: likelihood that an observed difference represents a real difference, rather than due to chance. Is weakened by bias and small sample sizes 

o  Validity: results are unbiased and give trust estimate of the measured effect. Extent to which a variable or intervention measures or accomplishes what it is supposed to. Does it measure what it claims to measure – described by specificity and sensitivity, etc


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