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 .