Transcript How to read a paper - Derby GP Specialty Training Programme

Medical Statistics
Half Day Release
11/1/12
Aims
 Overview of Evidence based practice
 Revision and Application of Medical
Statistics- Broad concepts
 .....Help with passing the AKT
 Principles of reading a Paper
 Interpretation and Application
Reading a Paper
&
Study Design
Critical Appraisal
Finding what is relevant, reliable and useful
What Influences our decisions?
Science/Social Science
Scientific Method
Counting differences
Understanding differences
Arts and humanities
Ethics
Adaptability, broad perspective
Humane Judgement
Technical judgement
CLINICAL DECISONS
Reading Papers
General Principles
• Clarity about the intent of reading that
paper i.e. ‘What Question am I answering’
• Find some Papers..
• Is the Study relevant?
– Population
– Intervention
– Condition
– Outcomes
What Questions to Ask
•
•
•
•
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Where?- Does it apply to my patients
Who?- Bias/Confounding factors/Sponsors
When?- Up to date/still applicable
Aims?
Demographics
– Inclusion criteria
– Exclusion Criteria
• Study Design
– Intervention
– Environment/Was it realistic
• Results
– What do they clain to have found- Is this reflected in statistics?
– Is it Relevant to your patients+clinical practice?
What is the Purpose?
• What question is the paper trying to
answer?
• Clear statement of intent
• Summary of background research related
to the topic
• Hypothesis presented in Methods section
• Trying to show a difference between 2
interventions
– Null hypothesis
What type of study was done?
Primary Vs Secondary Studies
– Experiments
• intervention in an artificial and controlled
environment
– Clinical Trial
• Intervention with follow up
– Surveys
• A measurement within a group of individuals
Secondary studies
• Overviews
– Reviews
• Summarise primary studies
– Guidelines
• Draw conclusions from studies
– Economic Analyses
• Draw conclusions regarding use of resources
– Systematic Reviews
• Rigorous+predefined methodology
– Meta-Analyses
• Integrated data from numerous studies
Was the Study design Appropriate?
What Field of research is it?
• Causation
– Establishing a link between two variables
– Cohort or Case Control studies, Case reports useful for rarer
diseases
• Prognosis
– What is likely to happen once disease picked up
– Longitudinal Cohort study
• Screening
– Picking up disease at pre-symptomatic stage on large scale
population
– Cross sectional survey
• Diagnostic
– Demonstrating whether a test is valid+reliable
– Cross sectional survey where new+gold standard are compared
• Therapeutic
– Testing drugs/procedures/interventions
Hierarchy of Evidence
1.
2.
Systematic reviews + Meta-Analyses
RCT- with definitive results
•
3.
RCT- with non-definitive results
•
4.
5.
6.
7.
CI not overlapping the threshold clinically significant effect
Point estimate suggestive of a clinically significant effect but
with CI overlapping the threshold for this effect
Cohort
Case-control
Cross sectional surveys
Case reports
RCT’s
• Random allocation to a
group for a particular
intervention.
• Followed up with regards
to outcomes which are
pre-defined at outset.
• Groups identical apart
form intervention..
• Thus establish a
causative relationship.
Advantages
• Evaluation of single variable in a specific
and precisely defined population group.
• Prospective outlook.
• Potentially eradicates bias by using
identical groups.
• Allows assimilation of data into MetaAnalyses.
Disadvantages
• Expensive, Time consuming
• RCT not done properly- small patient group/time
length
• Funding- Bias/research agenda
• Analysis of pre defined objective endpoints
– Excludes qualitative aspects which may be useful
• Publication Bias- Reporting specific results
• Limited Application to real life
– Strict inclusion criteria of set population
• Imperfect Randomisation:
– Failure to randomise Pts- Clinician offers study participation to
specific Pts only.
Cohort Studies
• 2+ groups selected according to differences in
exposure to particular agent.
• Followed up to see outcomes in each group.
• Long follow up period according to disease
development. Looks at aetiology not Rx.
• RCT Pts already have disease whereas Cohort
studies look at the development of disease
according to exposure
• Examples:
– Does high blood pressure get better over time?
– What happens to infants who have been born very prematurely,
in terms of subsequent physical development and educational
achievement?
Famous Study…
• Sir Austin Bradford Hill, Sir Richard Doll, and, Richard Peto:
• Followed up 40 000 British doctors divided into 4 cohorts
– Non-smokers, Light, Moderate, Heavy smokers
– Using both all cause mortality (any death)+ specific mortality (death
from a particular disease) as outcome measures.
– Publication of their 10 year interim results in 1964
– RESULTS:
– Showed substantial excess in lung cancer mortality
– ‘Dose-response’ relation (the more you smoke, the worse your chances
of getting lung cancer)
– Conclusion: Link between smoking and ill health was causal rather than
coincidental
– The 20 year and 40 year results (94% follow up of those recruited in
1951 and not known to have died) illustrate strength of evidence
obtained from Case Control study.
Case-Control Studies
• Pts with a particular disease are matched with control
equivalents.
• Retrospective collection of data regarding possible
exposure and causative factor.
• Examined Aetiology rather than Treatment intervention
• Useful for rarer conditions
• Does not show causality- an association between 2
variables does not prove a cause and effect relationship.
•
Examples:
– Does the prone sleeping position increase the risk of cot death (the
sudden infant death syndrome)?
– Does whooping cough vaccine cause brain damage?
– Do overhead power cables cause leukaemia?
Cross-sectional Surveys
•
•
•
A representative sample of Pts is interviewed to answer
a specific clinical question.
Data collected at single time point but often relating to
past experiences/events.
Examples:
– What is the “normal” height of a 3 year old child?
– What do psychiatric nurses believe about the value of
electroconvulsive therapy in severe depression?
– ..‘A doctor notices that two newborn babies in his hospital have
absent limbs (phocomelia). Both mothers had taken a new drug
(thalidomide) in early pregnancy. The doctor wishes to alert his
colleagues worldwide to the possibility of drug related damage
as quickly as possible’..
Case Reports
• Describes the Medical History of a single Patient
in narrative form.
• Often multiple run together to form Case series
where several Pts with same condition are
described to gain insight into particular aspect of
disease area.
• Qualitative, detailed info
• Time restricted
• low statistical hierarchy
Now.....Lets read a Paper
Clinical Efficacy of 3 common
treatments in acute otitis externa
in primary care: RCT
What Questions to Ask
•
•
•
•
•
Where?- Does it apply to my patients
Who?- Bias/Confounding factors/Sponsors
When?- Up to date/still applicable
Aims?
Demographics
– Inclusion criteria
– Exclusion Criteria
• Study Design
– Intervention
– Environment/Was it realistic
• Results
– What do they claim to have found- Is this reflected in statistics?
– Is it Relevant to your patients+clinical practice?
– Application...Explain the result to a worried Mother.