Observational Study Design

7/28/2019 Observational Study Design

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OBSERVATIONAL STUDY

DESIGN

Oleh :

Dr. Nurjannah, MPH


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Objectives

1. Describe each study design

2. State the advantages and disadvantagesof each study design

3. Understand the difference betweendescriptive and analytic studies

4. To be able to aply different study design

to the same research question5. Recognize each study design in medical

literature


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Type of Study

Epidemiological

Study

Observational

Study

Interventional

Study

Grup Individual

Descriptive or

Analitycal Descriptive Analytical

Ecological

Study

Cross

Sectional study

Cohort

Study

Case Control

Study


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Type of Studies

Observational Analytic Studies

Cross Sectional studies

Case Control studies

Cohrt Studies


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Cross Sectional

The simple form an observational

Both exposures and outcome are

measured at the same time

A snapshot of the health (or other)

experiences of the population at that

particular point in time


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Cross Sectional

Also called a survey (collection ofopinions) or pool (a questionnaireadministered to a sample of a people,

often about a single issue) Designed to determine what is

happening ? right now

Examines a characteristic or set ofcharacteristic in a set of subjects at onepoint in time ( prevalence)


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Advantages

Quick and inexpensiveFirst step for a cohort study

Able to yield prevalence estimates

Researcher has control over selection ofsubjects

Researcher a control over the mesurmentsused

Can study several factors or outcomes at theone time

Often provides early clues for hypothesis

generation & later more definitive study


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Disadvantages

Do not establish the true temporal sequence ofevents

They can only ascertain whether exposure is

associated with a given outcome; they cannotdetermine whether the exposure caused the

outcome

Potenstial bias in measuring exposurePotential sampling bias and/or survivor bias

They are not feasible for rare condition

Does not yield incidence or true relative risk


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Example Breast Feeding and

Obesity cross sectional study

Abstract :

Objective : to assess the impact of breastfeeding on the risk of obesity and risk of

being overweight in children attendingschool in Calima. In a sample of 80childen, early feeding, diet andlifestylefactors were assessedusingresponse to a questionnaire completed byparents

40


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Sample : 80 children aged 9 & 10, all Filipinos

Main Outcome Measure

Height and weight measurementBMI calculation : kg / height (m)2

Being overweight was defined as having a

body mass index above :male : 18.86 for 9 y/o

19.61 for 10 y/o


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Female : 19.2 for 9 y/o

20.2 for 10 y/o (NCHS) BMI for age

Questionnaire: Was your children breast

fed?, How long was your child exclusively

breast fed?

Other question : number of sibling, parents

ages and education, child health (LBW),

early feeding, frequency of eating selected

food


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Statistical Analyses

The prevalence of overweight and obese

children were calculated according to the

duration of breast feeding

X2 test (chi square) were used to compare

breastfed and nonbreastfed children and

their association to an obese / overweight

child.


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Result

The prevalences of obesity in children who

had never been breastfed was 4.5 % as

compared with 2.8% in breast fed children.

A clear dose response effect was identifiedfor the duration of breast feeding on the

prevalence of obesity


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The prevalence was 3,8 % for 2 month of

exclusive breast feeding, 2.3 % for 3-5 month,1.7 % for 6-12 month, and 0.8% for more than12 month

Breast feeding as a significant protective factor

against obesity development Similar relations were found with the prevalence

of being overweight

The protective effect of breast feeding was notattributable to differences in social class orlifestyle


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Conclusion

Prolonged breast feeding may helpdecrease the prevalence of obesity inchildhood

Since obese children have a high risk ofbecoming obese adults, such preventive

measures may eventually result in areductin in the prevalence ofcardiovascular diseases related to obesity


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Case Control Study

A case-control study is distinguished bythe fact that subject are selected on thebasis of whether or not they have the

disease (or other outcom) of interestCase (person/group with a given disease)are then compared to control (person/group without the given disease) in termsof their history of exposure to ahypothesized causal factor


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Case Control Study

Ascertain of exposure of background of

the two group and compare the proportion

Best suited for study of diseases where

medical care usually sought (hip fracture,

cancer) bacauses it make to easier to

identify cases


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Stucture

Exposure Outcome Design

Exposed a

Not exposed b

Exposed c

Not exposed d

Population

Case (peoplewith desease)

Control (people

without desease)

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Selection of cases

Ideally, investigator identifies & enrolls allincident cases in a defined population in aspcified time period

Selected cases from registries or hospital,clinics

Whenn all incident cases in population are

included, the study is a representative;otherwise there is potential for bias (e.g.referal bias


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Selection of Controls

Critical that exposure in the control is

representative of the exposure in the

population

Ideal controls would have same/similar

characteristics as the cases

Matching case control


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Population-Based Controls

The best control group is a random

sample of individuals from same source

population (as the cases) who have not

the developed the disease

Population-based controls are the best

way ensure that the distribution of

exposure among the controls isrepresentative


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Hospitals Control

Hospital control are the most frequently usedsource

Hospital control may not be representative of

exposure rates in the target populationThe use of other ill as person control will

provide a valid result only if their illness is

unralated the exposure in question


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Benefit of Using Hospital Controls

Convenient

Cheap

NumerusAvoids non-response

When a population-based case registry is

not available, hospital control betterrepresent the subpopulation from wich the

cases arose


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Other Controls

Neighborhood controls are somewhat

matched on SES & environmental

exposure but may overmatch & be

expensive

Friends & relatives also cause problems

with overmatching with habits,

environment and ccupation & aregenerally a poor choice for controls


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Use of Multiple Controls

Case to control ratio used is usually 1:1; if

large number and cost is the same for

both group

If a study has a small number of cases,

increasing the number of controls increass

power of study


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Advantages

Relatively shorter time and inexpensiveGood for desease with long latency

Valuable for studying rare or uncommon

conditions

Multiple etiologic factors evaluated for single

deseases

Well suited for studying disease with longinduction period

A relatively small number of subjects are

required


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Disadvantages

Inefficient if the exposure is very rare

They are limited to one outcome variable

Incidence rates or absolute risk estimatescannot be directly derived from them

Do not establish the temporal sequence of

event; in some situations the temporal

relationship between exposure and

desease may thus be difficult to establish

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Disadvantages

Prone to bias (selection of cases andcontrols recall, misclassification)

Difficult to determine representativeness of

case and controlsUnless study is population based cantmeasure incidence of desease

Bad for rare exposure (despite a largenumber of case, may still end up with fewexpossed cases)


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Cohort Studies

An observational research design which

begin when a groupof people (a cohort)

initially free of desease, are classified

according to a given exposure, and thenfollowed up over time


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Cohort Stucture

Exposure Outcome Design

Disease a

No disease b

Disease c

Not disease d

Population

(People withoutdisease)

Exposed

Not

exposed


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Type of Cohort

1. Prospective

Fixed

Open or Dynamic

2. Retrospective


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Prospective Cohort

Fixed Cohort Study

When the exposure groups in a cohort

study are defined at the onset of the study

without movement of individuals between

exposure groups, the exposure group are

reffered to as fixed cohorts (occupational.

War)


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Structure

Prospective fixed cohort study

Exposure Disease

?

?


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Prospective Cohort

Open or Dynamic Cohort Study

The other type of prospective cohort study

is the open or dynamic cohort study where

in individuals can be unexpose at one time

and unexposed at another time. The

person time analysis can take this into

account in calculating incidence densities


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Structure

Prospective dynamic (open) cohort study

Exposure Disease

?

?


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Retrospective Cohort

The point of initial exposure occurred

some time in the past and the experience

of the population is followed up to the

present time

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R t ti C h t St t


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Retrospective Cohort Structure

Prospective fixed cohort study

Exposure Disease

?

?= Present Basic on which group are selected at

= Absent beginning of study

? = To detemined


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Advantages

Provide strong information about thecausation of disease

Provide the measurement of the risk of

developing diseaseExposure can be measured without bias,because at the same time the outcomes

sre known; known confounder can bemeasured (especially in a prospectivestudy)


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Advantages

Can be used to examine multipleoutcomes

A range of factor that may influence the

outcome (e.g. smoking) can be measuredSuitable for examining the effect of rareexposures because this group can be

preferentially recruited at the baselineAllows the incidence of the disease to beestablished

Di d


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Disadvantages

Costly and time consumingMay be difficult to accurately define andmeasure exposure in some circumstances

Losses the follow up are not uncommon andmay introduce serious bias

Information bias may very in its effect over thecourse of data collection due to sometime

subtle drifting of the quality of data collectionUse of the retrospective design is only possibleif historical data of adequate quality are

available

Rancangan Penelitian


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Rancangan Penelitian

Retrospective

p

o

p

u

l

a

s

i

Efek (+)Retrospektif

Efek (-)Retrospektif

F. Resiko (+)

F. Resiko (-)

F. Resiko (-)

F. Resiko (+)


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Rancangan Penelitian Cross Sectional

Populasi /Sampel

Faktor Resiko (-)Faktor Resiko (+)

Efek (-)Efek (+)Efek (-)Efek (+)


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TERIMA KASIH


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Observational Study Design

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