overview of epidemiological study
DESCRIPTION
TRANSCRIPT
Design 1-11-03 11 NOVEMBER 2003
Overview of Epidemiological Study
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Outline
• Step of Study
• Formulate Questions
• Type of Designs
• Observational study
• Experimental study
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Main Step of Epidemiological Study/ Research Process
Formulate the PROBLEM(S)
Research DESIGN
Collect Data
Analyze Data
Interpret the Results
Communicate the Results
Review Literature
Framework Theory
Formulate HypothesisObjective
Define Target PopulationIntervention (if any)
Define VariablesDetermine Measurement
Methodology
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Types of Questions
• Describing natural events
• Assessing association of observed interesting factors and outcomes
• Evaluating effect of assigned interesting factor on outcome
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Type of DesignsType of Designs
ExperimentalExperimental
ObservationalObservational
DescriptiveDescriptive
AnalyticalAnalytical
Cohort(Prospective ; E -> O)
Cohort(Prospective ; E -> O)
Case-control(Retrospective ; O -> E)
Case-control(Retrospective ; O -> E)
Cross-sectional(Concurrent ; E/O)
Cross-sectional(Concurrent ; E/O)
No comparison group
With comparison group
Assigned exposure
Natural exposure
E=ExposureO=Outcome
X-sectional
longitudinal
Design 1-11-03 61 NOVEMBER 2003
“Exposures” and “Outcomes”
Experimental studies - Investigator assigns exposure- Investigator assesses outcome
Experimental studies - Investigator assigns exposure- Investigator assesses outcome
Observational studies - Individual assigns exposure- Investigator assesses outcome
Observational studies - Individual assigns exposure- Investigator assesses outcome
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Results
QuestionsQuestions Studyprocedures
Studyprocedures
Results(Data or Fact)
Results(Data or Fact)
True valueTrue value ErrorError+
Random error(Chance)
Random error(Chance)
Systematic error(Bias)
Systematic error(Bias)
Proper statisticsProper statistics Proper design and methodology
Proper design and methodology
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BIAS
Occur anywhere within the research processOccur anywhere within the research process
Systematic deviation from the truth thatdistorts the results of research
Systematic deviation from the truth thatdistorts the results of research
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CONFOUNDING
ConfounderConfounder
Interest association
Junk Food
Smoking
CA Colon
ExposureExposure OutcomeOutcome
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CONFOUNDING
The distortion of the apparent effect of an exposure on risk brought about by the association with other factors that can influence the outcome
e.g. effect of coffee drinking on myocardial infarction is measures as estimated relative risk of 1.8The specific relative risk in non-smokers is 1.2The specific relative risk in smokers is 2.7
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Random error
A wrong results due to chance,- unknown sources of variation being equally likely to distort the findings in either direction
The effect of random error can be reduced by recruiting larger numbers or by adjustment for known confounders in the analysis
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Study Designs
ObservationalObservational
DescriptiveDescriptive
No comparison group
Natural exposure
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Descriptive Study
A study concerned with and designed to describe the existing distribution of interest outcomes, without regard to causal or other hypotheses
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Example of descriptive study
The Epidemiological Study of Teenage Pregnancy
In Malawi, during 2000-2008
The Epidemiological Study of Incidence of Lung Cancer
In Thailand during 2000-2005, and major Risk Factors
DISCUSSION
Structure of descriptive study
Population; the group to be concluded by the study results
Sample
Random
Cross-sectional OR Longitudinal
-main outcome -other interested outcomes
Selection bias
Measurement bias
Follow-up bias
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Limitation
• Association found can not be infer directly to the population
Descriptive Study
Advantages
• Cheap
• Finding can be used to generate research hypothesis for further higher designs
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Type of Designs
AnalyticalAnalytical
With comparison group
ObservationalObservational
DescriptiveDescriptive
No comparison group
Natural exposure
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ANALYTICAL STUDY
EXPOSURE EXPOSURE OUTCOMEOUTCOMECOHORTCOHORT
CASE - CONTROLCASE - CONTROL
CROSS-SECTIONALCROSS-SECTIONAL
PRESENT FUTUREPAST
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COHORT STUDY
CohortSelected for
study
Exposed group
Non-exposed group
With outcome
Without outcome
With outcome
Without outcome
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Pregnant women during
1951-1970
Pregnant women with hypertensive disease
Pregnant women without hypertensive disease
Having circulatory diseaseNo circulatory disease
Having circulatory diseaseNo circulatory disease
1999
COHORT STUDY
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Select groups in 2003
Trace
1968 2003 2028
Retrospective
COHORT STUDY DESIGNS
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Select groups in 2003
Follow
1968 2003 2028
Prospective
COHORT STUDY DESIGNS
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Select groups in 2003
Follow
1968 2003 2028
Prospective
COHORT STUDY DESIGNS
Follow
Retrospective
Historical prospective
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ADVANTAGES
• Prospective data collection
• Temporality (time-sequence)
• Allows for calculation of incidence outcomes in exposure and non-exposure individuals
• May be used to study multiple outcomes
• Efficient for studying rare exposures
• Minimize recall bias
COHORT STUDY
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LIMITATIONS
• Tend to be expensive (large sample size) and time consuming (long follow-up period)
• Possibly having non-comparative group
• Loss to follow-up
• Ineffecient to study rare outcome
COHORT STUDY
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Cautions when implementing a cohort study
• Chance of outcome have to be occurred among comparative groups
• Exposure and non-exposure groups are clearly different but all other factors be very similar ( to reduce selection bias)
• Outcome must be measured by the same tool and procedure among the groups
• Having a good plan for preventing lost to follow-up
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Plan for implementing a cohort study
Title: Passive Smoking and risk of coronary heart disease in women: prospective cohort study
BMJ 1998;317:1341–5Objective:To examine the relation between passive smoking and risk of coronary heart disease in a cohort of women from the ALSPAC
Population: women from 34 to 59 years of age without previously diagnosed coronary heart disease,stroke, or cancer at baseline in 1980. They were classified to four categories for passive smoking, included; almost never, 1 3 times per month to once per week, 2 4 times per week, and >5 times per week. These categories were represented 4 groups of exposure.
Design 1-11-03 281 NOVEMBER 2003
• The main outcome:Incident coronary heart disease, defined as non fatal myocardial infarction or fatal coronary heart disease occurring after the return of the 1980 questionnaire but before 1 June 1994.
• Measurement:The medical records of women who reported a non fatalmyocardial infarction were reviewed by physiciansblinded to self reported risk factor status. Non fatalmyocardial infarction was confirmed in accordancewith the criteria of theWorld Health Organisation pluseither diagnostic electrocardiographic changes orelevated serum concentrations of cardiac enzymes.
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Potential confounding variables including age, body mass index, hormones for treating the menopause, alcohol, multivitamin, and vitamin E supplements were updated every two years. Aspirin use was assessed in 1980, 1982, 1984, and 1988. Vigorous exercise was assessed in 1980.
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Statistical Analysis:The relative risk was computed as the rate in a specific category of passive smoking divided by that in the lowest category (almost never), with adjustment for age in five year categories.
In multivariate analyses, intake of total energy, fat intake, and other potential confounding variables were simultaneously included .