study designs. kate o’donnell general practice & primary care
TRANSCRIPT
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Study designs.
Kate O’DonnellGeneral Practice & Primary Care
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Epidemiological questions.
How common is the problem?
What are the risks of further problems?
Are treatments effective?
What are the causes of the problems described?
How effective are services?
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Underpinning concepts.
What concepts underpin these types of questions?
Risk
EffectivenessAssociation
Likelihood
Population level data applied to individuals
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What can studies do?
Describe the situation: Descriptive.
Explain the situation: Analytical.
Compare approaches: Experimental.
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Study designs.
Descriptive
Cross-sectional, longitudinal.
Analytic
Case-control studies.
Cohort studies.
Quasi-experimental
Natural experiments, policy interventions.
Experimental
Randomised controlled trial.
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Type of Study Descriptive Analytical Experimental
Case study Yes No No
Case series Yes No No
Cross-sectional Yes Yes No
Case-control Yes Yes No
Cohort Yes Yes No
Natural experiment Yes Yes Quasi
Randomised control trial Yes Yes Yes
Study designs
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Prevalence Cross-sectional
Cause/Aetiology
Cross-sectional;Case-control;Cohort.
Prognosis Cohort.
Harm Case-control;Cohort.
Effectiveness Randomised controlled trial.
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Present FuturePast
Cross-sectional
Case-control
Cohort
From Altman. Practical Statistics for Medical Research, 1991.
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Sample
Randomise
Intervention group
Control group
Randomised controlled trial
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Case control studies: Key features.
Retrospective, i.e. backward looking.
Relate an effect or outcome to a probable cause.
Individuals with disease (cases) compared to individuals without disease (controls).
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Case control study.
From: http://library.downstate.edu/EBM2/2500.htm
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Advantages & disadvantages
Advantages Disadvantages
Quicker than cohort studies No measure of incidence
Relatively small study population required
Can only investigate one disease at a time & no temporal association
Good for rare diseases – can select all known cases
Retrospective, so data may be biased, e.g. poor records; memory recall
Case selection bias – must define cases very explicitly
Control selection bias
Matching controls to cases can be difficult
Can only determine the odds ratio, not relative risk
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Appraising case control studies
Methodological approach:
Are cases and controls similar, except for exposure to the putative cause?
Is collection of retrospective data objective?
Is there evidence of “causation”?
Statistical reporting:
Type of data – influences statistical analysis.
Reporting of risk.
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Cohort studies: Key features.
Prospective, i.e. forward looking.
Follow a population sample over a period of time, often years.
Record new disease events, i.e. the disease incidence.
Cohorts can be selected in either of two ways.
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Selection of a cohort: External controls.
i. A cohort of individuals that have been exposed to a postulated causal factor of disease. The cohort is followed up and all new cases of the disease recorded. In this type of study, the exposed group would be compared to a matched, unexposed group and the incidence of disease in the two groups compared, i.e. an external control.
ii. An example would be a study to record the incidence of mesothelioma in factory workers exposed, or not, to asbestos.
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Cohort studies.
From: http://www.socialresearchmethods.net/tutorial/Cho2/cohort.html
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Selection of a cohort: Internal controls.
A population cohort can be selected and followed up over many years to determine the incidence of disease(s). In this type of study, those in the cohort who do not develop the disease act as the controls for those who do, i.e. an internal control.
An example would be the MIDSPAN study.
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Cohort studies.
From: http://www.socialresearchmethods.net/tutorial/Cho2/cohort.html
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Advantages & disadvantages
Advantages Disadvantages
Calculate incidence Need a large population
Don’t have to select controls Follow-up can take many years, so expensive
Prospective data collection, so less bias
Not good for rare diseases
Can study more than one disease or outcome
Lose individuals in follow-up
Temporal association Potential for bias in recruitment, assessment and follow-up.
Standardised assessment Assessment criteria may “drift” with time
Can estimate relative risk May become irrelevant before the end of the study
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Appraising cohort studies
Methodological approach:
Representativeness of the inception cohort.
Characterisation at study baseline.
Nature and completeness of follow-up.
Statistical reporting:
Type of data – influences statistical analysis.
Reporting of risk.
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Prevalence Cross-sectional
Cause/Aetiology
Cross-sectional;Case-control;Cohort.
Prognosis Cohort.
Harm Case-control;Cohort.
Effectiveness Randomised controlled trial.
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Randomised controlled trials
A clinical trial in which:
• at least two treatments, programmes, interventions are compared.
• one of these is a control group.
• allocation uses a random, unbiased method.
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Population
Group 1
Group 2
Outcome
Outcome
New treatment
Control treatmentFrom: Critical Appraisal Skills Programme (CASP), Oxford.
Randomised controlled trials
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Explanatory trials
Measure efficacy:
the benefit a treatment produces under ideal conditions.
e.g. Phase III drug trials.
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Pragmatic trials
Measure effectiveness:
the benefit a treatment produces in routine clinical
practice.
Aim to inform choices between treatments.
Patients should be analysed in the group to which they were initially randomised, i.e. intention to treat analysis.
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Intention to treat analysis
All patients allocated to one arm of a RCT are analysed in that arm, whether or not they completed the prescribed treatment/regimen.
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Two by two table
Outcome event Total
Yes No
Experimental group
a b a + b
Control group c d c + d
Total a + c b + d a + b +c + d
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Appraising RCTs
Methodological approach:
Was assignment to the different groups randomised?
Was the randomisation process/list concealed?
Was everyone who entered the trial accounted for at the end?
Were subjects and assessors “blind” to treatment allocation when assessing outcomes?
Were groups similar at start of trial and treated similarly throughout the study?
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Appraising RCTs
Statistical reporting:
Were subjects analysed in the group to which they were randomised: intention to treat analysis.
Type of data – influences statistical analysis.
Reporting of risk: RRR vs ARR.