choosing your study design
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Choosing your study designTRANSCRIPT
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Choosing the appropriate study design
Research Week 2014Health Cluster UKM
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USUAL STUDY DESIGNS
OBSERVATIONAL STUDY
EXPERIMENTAL STUDY
DESCRIPTIVE STUDY
ANALYTIC STUDY
CROSS-SECTIONAL
CASE CONTROL
COHORT
CLINICAL TRIAL
COMMUNITY TRIAL
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Difference Between Study Designs
Study Design Intervention Temporal Sequence Sampling
Cross Sectional AbsentRisk Factor and Outcome at same time.
Yes
Case Control AbsentOutcome first, then risk factor (retrospectively).
Matching
Cohort AbsentRisk Factor first, then Outcome.
Maybe
Clinical/ Community Trial
PresentIntervene, then measure Outcome.
Randomisation
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Which design is appropriate for your study?
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Usual study design
Commonly chosen in PPUKM
• Clinical trial• Cross-sectional • Case-control• Cohort (PURE &
Malaysian Cohort)
Specific for Patho/Diagnostic
• Diagnostic testing• Sensitivity,
Specificity & ROC• Kappa/ Agreement
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Which one?
• If you have an intervention, then it is a clinical trial.
• If there is no intervention, then it is one of the observational studies, usually cross-sectional.
• If the disease is rare, less than 5% prevalence rate, then it is a case-control study.
• If you are looking forward in time for new incidence of the disease, then it is a cohort study.
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Clinical Trials
If you have an intervention, then probably it is a clinical trial.
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Clinical Trial
Time
Intervention
Confounders
OutcomeLook forward in time
Starts with Intervention, then measure the future Outcome
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EXPERIMENTAL STUDY
Eligible Participants
T +
T -
C +
C +
C -
C -
Future
Randomisation
Study Population
Selection
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Clinical Trial
Drug Sogood
Drug Feelgood
No improvement (25%)
Improved (75%)
Improved (70%)
No improvement (30%)
Sample ratio (1:1)Depressed
patients
Intervention Outcome-Improved?
Starts with Intervention, then measure the future Outcome
Time Future
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PROCEDURE• Assignment of subjects to treatments• Interview and examinations• Methods of assessment• Lab. Studies• Treatment schedules: number of units per visit,
rules for changing dosage, compliance checks• Adverse reactions: definition and grading, inquiry,
management.• Drops out: definition, handling and recording,
terminating and extending study
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CROSS-SECTIONAL STUDY
Also known as Prevalence Study or Survey. Easiest study design.
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Cross-Sectional Study
• Measures the relationship of variables in a defined population at one particular time
• Both risk factors (exposure) and disease outcome are observed at the same (point in) time in a sample (or the entire population) of subjects.
• i.e. Studying the effect of overweight on the prevalence rate of diabetes mellitus.
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Exposure & Outcome
Time
Risk Factor
Confounders
Outcome
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Cross-Sectional Study
Time
Risk Factor
Confounders
Outcome
Both Risk Factor & Outcome measured at the same time.
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Cross-Sectional Study
Overweight (40%)
Normal (60%)
Disease - (68%)
Disease + (32%)
Disease + (8%)
Disease - (92%)
Sample ratio
Risk Factor -Overweight Outcome-Diabetes Mellitus
Both Risk Factor & Outcome measured at the same time.
Time
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Reminder
• If the prevalence/incidence rate is below 5%, please do not choose a cross-sectional study design. Your number of cases would be too low to do any analysis.
• For example, doing a cross-sectional study of HIV cases among Malaysian pregnant mothers. The rate of HIV+ is only 0.04%. Therefore even with 10,000 samples, you will only get 4 HIV+.(http://www.moh.gov.my/attachments/3886.pdf)
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Imagine a result like this
HIV+ HIV- TotalRisk Factor + 3 97 100Risk Factor - 1 9899 9900
4 9996 10000
Not much analysis could be done with a result like this. So instead do a case-control study for rare diseases.
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CASE-CONTROL STUDY
For rare diseases. Consider if prevalence rate below 5%.
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CASE CONTROL STUDY- CONCEPT
• comparison of group of diseased person (cases) with another group of non-diseased person (control) for past exposure to a suspected cause of the disease.
• arises because of hypothesis that the risk factor (exposure) causes the disease
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Case-Control Study
Time
Exposure
Confounders
Outcome
Look back in time
Starts with Outcome, then trace the retrospective exposure
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Case-Control Study
Cataract
Normalvision
DM - (50%)
DM + (50%)
DM + (8%)
DM - (92%)
Sample ratio (1:1)
Outcome-Cataract Risk Factor-Diabetes Mellitus
Starts with Outcome, then trace the retrospective exposure
Time Past
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CASE SELECTION• Determine clear and reproducible definitions of
the health problems to be studied (avoid misclassification bias)
• source of cases All persons with the disease seen in particular
facility(ies) in a specified period of time. All persons with the disease found in general
population.
• Incidence cases (newly diagnosed cases) preferred
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CHOICE OF CONTROLS• Controls should ideally be selected from the same
population gave rise to cases• Similar to cases in regard to past potential exposure• Free from study disease• If controls are patient with other diseases then
select only diseases that are not known to have relationship with factors under study.
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ADVANTAGES
• able to study rare diseases• can explore multiple exposures• relatively inexpensive• can calculate Odds Ratio• can support causation but not prove it• easy to get cases
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COHORT STUDY- ignore, none of you will do this.
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COHORT STUDY
BASIC CONCEPT• Group or groups of individuals are
studied over time as to onset of new cases of disease and factors associated with the onset of disease.
• Synonyms : incidence study, longitudinal study, prospective study.
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Cohort Study
Time
Exposure
Confounders
OutcomeLook forward in time
Starts with Risk Factor, then measure the future Outcome
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COHORT STUDY
DISEASE
Free from disease
EXPOSED GROUP
UNEXPOSED GROUP
DISEASE
NONDISEASE
NONDISEASE
FOLLOW-UP
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Cohort Study
Normal
Overweight
DM - (68%)
DM + (32%)
DM + (7%)
DM - (93%)
Sample ratio (1:1)
Risk Factor-Weight Outcome-Diabetes Mellitus
Starts with Risk Factor, then measure the future Outcome
Time Future
Free from DM
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Recruitment
• Those recruited must be free from the disease of interest at the beginning of the study.
• Those with sub-clinical presentations of the disease may miss from being excluded. This is one of the challenges.
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Follow-up
• Keep participation at > 90%• Must have equal ability to detect disease in all
subjects and all groups, with standard measurement• Active vs Passive follow-up• Verbal Autopsy• Blinding of the assessor• Assess both primary and secondary outcomes
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Next
Calculate sample size