Descriptive Epidemiology & Descriptive Epidemiology &

Study designStudy design

Potjaman Potjaman SiriarayaponSiriarayaponBureau of EpidemiologyBureau of Epidemiology

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Outline

General concept of descriptive epidemiology

Study design in epidemiologyDescriptiveAnalytic

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KEY CONCEPTS

““Descriptive epidemiologyDescriptive epidemiology”” is the first step in epidemiological study. Careful observation of available information alone have led many successful preventive measures in the past.Describing health or disease occurrence according to place, person and time can lead to very meaningful hypothesis formulations.

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TIME

Disease rates change over time. Some of these change occur regularly and can be predicted.

By examining events that precede a disease rate increase or decrease, we may identify causes and appropriate actions to control or prevent further occurrence of the disease.

5

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

1 9 6 0

1 9 6 5

1 9 7 0

1 9 7 5

1 9 8 0

1 9 8 5

1 9 9 0

Year

Reported cases (per 1,000)

Vaccine Licensed

Measles by year of report, United States, 1960-1989

Secular (longSecular (long--term) trends:term) trends: to predict or evaluate control program

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Seasonality:Seasonality: to see seasonal pattern to see seasonal pattern over yearsover years

0

2

4

6

8

1 0F e

b

Ap r

Jun

Aug O

c t

De c

9 79 89 9

Cases of eosinophilic meningitis, Nan province,Thailand, 1997-1999

Cases

Onset by Month

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Day of week and time of day:Day of week and time of day: especially especially important for condition that related to important for condition that related to

occupational or environmental conditionoccupational or environmental condition

0

5

1 0

1 5

2 0

2 5

3 0

8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8

Deaths

Time of Day

Fatalities associated with farm tractor injuriesby time of day, Georgia, 1971-1981

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Epidemic period:Epidemic period: to show the time course to show the time course of disease outbreakof disease outbreak

0

10

20

30

40

19 21 23 25 29 1 3 5 7 9 11 13 15 17 19 21

FloodCases

Date of onsetNumber of leptospirosis patientsby date of onset, Nov 25-Dec 21, 2000

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PLACE

We describe a health event by place to gain insight into the geographical extent of the problem.

We may use place of residence, birthplace, place of employment, school district, hospital unit, etc., depending on which may be related to the occurrence of the health event.

MyanmarUmphangdistrict

Nov.-Dec.

Jan-Feb.

Mar.

villageRefugeecamp

Meningococcal cases,Meningococcal cases,TakTak province, province, 20022002--2003 2003

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Thai-Myanmar border area in Umphangdistrict

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In the refugee camp

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House of Karen people outside the camp

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PERSON

There are several person categories available:

inherent characteristics: age, sex, race

acquired characteristics: marital status, immune

activities: occupation, use of medication/tobacco/drugs

condition under which their live: socioeconomic status, access to medical care

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Sex specific attack rates of reported Sex specific attack rates of reported leptospirosisleptospirosis casescases,, SongklaSongkla,, NovNov 2525--DecDec15, 200015, 2000

5 9 . 5

2 0

0

2 0

4 0

6 0

8 0

M a l e F e m a l e

S e x

C a s e s / 1 0 0 , 0 0 0

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Age Age specificspecific attack rates of attack rates of reportedreportedleptospirosisleptospirosis casescases,, SongklaSongkla,, NovNov 2525--DecDec 15, 15, 20002000

21.6

33.9 37.3

51.140.7

58.3

16.7

0

20

40

60

80

0-9 10-19 20-29 30-39 40-49 50-59 60+

Age group

Cases /100,000

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DEFINITION OF EPIDEMIOLOGYDEFINITION OF EPIDEMIOLOGY

"Epidemiolgy is the study of the distribution and determinants of health-related states or events in specified populations and the application of this study to the control of health problems"

(John M. Last, 1988)

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Study designStudy design

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Why Why ““type of studytype of study””??

•Design more appropriate studies to get right answers to the specific questions

• Getting more optional study design to the questions

• Shape up proposals for getting more valid results

TYPES OF OBSERVATIONAL STUDIES

DescriptiveStudies

Little is knownabout theoccurrence, ordeterminants of thedisease

- Identify cases, estimate diseasefrequency, examine time trend

- Justify additional (analytic)studies to test hypotheses

AnalyticStudies

Enough is knownabout the ddisease,specific hypothesescan be tested

- Test specific etiologic hypotheses

- Justify additional studies

- Suggest potential for diseaseprevention

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STUDY DESIGN OF DESCRIPTIVE

STUDY

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STUDY DESIGN OF DESCRIPTIVE STUDYSTUDY DESIGN OF DESCRIPTIVE STUDY

Individual levelCase reports or case seriesCross-sectional surveys of individual

Population levelEcological study (Correlational study)

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CASE REPORTS AND CASE SERIESCASE REPORTS AND CASE SERIES

Case reports Case reports :: describe experience of a single patients. Case reports document unusual medical occurrence and can represent the first clues in the identification of new diseases.

Case series Case series :: collections of individual case reports. Investigation of the activities of the individual in case reports can lead to formulation of a hypothesis.

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Streptococcus suis

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CASE REPORTS AND CASE SERIES

AdvantageAdvantagediscover new diseases

bring background information to form hypothesis about risk factors

DisadvantageDisadvantagemight base on only one casedo not have a comparison group

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CROSSCROSS--SECTIONALSECTIONAL STUDYSTUDY

Total population

Cases

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Character of cross-sectional study

Collect data for all population; case & healthy people

Information obtain is the number of total cases among total population at that study period: prevalence

Can be either descriptive or analytic, depend on design

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CROSS-SECTIONAL STUDY

Descriptive Descriptive Collected number of cases and number of total population

Can assess only prevalence of disease or other health events, also called prevalence study

AnalyticAnalyticExposure anddisease status areassessedsimultaneously

Can determine association between exposure and disease

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Vaccine coverage among Thai children

Proportion of children received vaccine

Education level of mother and vaccine status in children

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CrossCross--sectional study sectional study

Defined Population

ExposedExposed

Have disease

Not exposed:

Have disease

NotExposed:

Do nothave disease

ExposedExposed

Do nothave disease

Gather Data on Exposure and Disease

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Hypothetical illustration of the interrelationship between an occupational exposure and prevalence of disease

Job A Job A ((hazardoushazardous))

100100WorkersWorkers

80 well

20 ill

Job B (non-hazardous)

100Workers

95 well

5 ill

80 well

1010 illill

10 ill

95 well

1515 illill

10 change jobsdue to illness

Point X

Prevalence of job A = 20 % ( 20/100 )Prevalence of job B = 5 % ( 5/100 )Prevalence ratioPrevalence ratio == 44

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Hypothetical illustration of the interrelationship between an occupational exposure and prevalence of disease

Job A Job A ((hazardoushazardous))

100100WorkersWorkers

80 well

20 ill

Job B (non-hazardous)

100Workers

95 well

5 ill

80 well

1010 illill

10 ill

95 well

1515 illill

10 change jobsdue to illness

Point YPrevalence of job A = 11 % ( 10/90 )Prevalence of job B = 14 % ( 15/110 )Prevalence ratioPrevalence ratio == 0.80.8

AdvantageEasy and less time consumedEstimate magnitude of problem especially for rare diseaseFirst step for exploring and forming hypothesisAble to use data for further case-control or

cohort during analytic study

DisadvantageDifficult to confirm that risk factor come before disease (temporal relationship)Length biased sampling: Disease that has long duration will over-represent the magnitude of illness while short duration will under-represent illness.

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STUDY DESIGN OF DESCRIPTIVE STUDYSTUDY DESIGN OF DESCRIPTIVE STUDY

Individual levelCase reports or case seriesCross-sectional surveys of individual

Population levelEcological study (Correlational study)

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ECOLOGICAL STUDYECOLOGICAL STUDY

These studies are an extension of the use of routine data.

Essentially the average exposure of the population is plotted against the rate of the outcome for that population

This is done for several populations and the data are then examined for evidence of an association between exposure and outcome.

Group of Proportion of Suicide rate Prussian Province Protestant (per 100,000 pop.)

A 9.56 0.30

B 16.36 0.45

C 22.00 0.79

D 26.46 0.95

Study to find relationship between suicide rate and proportion of Protestant

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0

0.2

0.4

0.6

0.8

1

0 10 20 30

A

B

C

D

Prov. Proportion of SuicideProtestant rate

A 9.56 0.30

B 16.36 0.45

C 22.00 0.79

D 26.46 0.95

Suicide rate (per 10 5/year)

Proportion of Protestant (%)

Correlation between proportion of protestant and suicidal rate in 4 groups of Prussian provinces

Inci

denc

e

Average temperature15.3 24.3

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14.34

Correlation of JE incidence and average temperature in Dali prefecture,

Yunnan province, China, 2001

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Rationale for ecological studyRationale for ecological study

Low cost and convenience: make use of existing data

Measurement limitations of individual-level studies: environmental study

Interest in ecological effect: social norm,population intervention

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ECOLOGICAL STUDYECOLOGICAL STUDY

AdvantageAdvantage:: cheap, quick and convenient since it usually come from existing data

DisadvantageDisadvantage::

inability to link exposure with disease in individual (ecological fallacy)limit to control effect of other factors

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In summary

• Ecological study is a convenient, cheap and simple study.

• Unit of study is the aggregate data not individual level.

• It is usually be conducted as the first step study for research.

• The result is difficult to interpret because of confounding and bias.

Ecological or correlation study Ecological or correlation study

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Study design

Observational Experimental

Descriptive

Case and serial case

Cross-sectional

Ecological

Analytic

Case-control

Cohort

Quasi-experimental

Experimental

Cross-sectional

Ecological

Summary of the study designSummary of the study design

Analytic study

Disease among exposed?

Disease among non-exposed?

UsuallyUsually prospectiveprospective

Populationat risk

Cohort studyCohort study

Exposed

Not Exposed

and

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Distribution of illness according Distribution of illness according to exposure in a cohort studyto exposure in a cohort study

Exposed

Not exposed

Ill Not ill

a b

c d

a+b

c+d

RiskRisk

a+b

c+d

a

c

Relative risk = Risk Relative risk = Risk exposedexposed / Risk / Risk not exposednot exposed

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Key featuresShould have follow up period (even in retro-cohort)

Have confine population

Comparison group should be as similar as possible

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Cohort study Cohort study

AdvantageAdvantage

Determine multiple effect of single exposureAble to estimate incidenceMinimize bias for prospective study

DisadvantageDisadvantageExpensive and time consumedValidity of result depend on follow up

Disease(Case)

Exposure

?

No disease(Control)

?

CaseCase--control studycontrol study

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Distribution of cases and controls according Distribution of cases and controls according to exposure in a caseto exposure in a case--control studycontrol study

Exposed

Not exposed

Total

Cases

a

c

a+c

Odds of exposure a/cOdds of exposure a/c

Controls

b

d

b+d

b/db/d

Odds ratio = OddsE (cases) / OddsE (controls)

Odds

Team A plays 10 games:8 wins2 defeats

Risk (probability) of losing = 2/10Odds of losing = 2/10 = 2/8

8/10

Probability of eventProbability of non-event

Key features

Study only some part of the population, especially among non-case

Case and control should come from the same source population, to allow possibility that control could receive the exposure

Case and control are not necessary to be the same

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Advantage Advantage Cheap, quick and efficient for rare Cheap, quick and efficient for rare diseasediseasePotential role for testing multiple risk Potential role for testing multiple risk factorsfactors

DisadvantageDisadvantageSusceptible to biasSusceptible to biasInefficient for rare exposureInefficient for rare exposureSometime difficult to establish Sometime difficult to establish temporal relationship between exposure temporal relationship between exposure and diseaseand disease

CaseCase--control study control study

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Advantage:Advantage:•• Feasible and practical especially for Feasible and practical especially for ethical concernethical concern•• Study population is more represent of Study population is more represent of the target population the target population

Limitation: Limitation: •• Difficult to replicate Difficult to replicate •• Less control of extraneous factors that Less control of extraneous factors that will lead to distortion of the resultwill lead to distortion of the result•• Less secure to make generalization Less secure to make generalization

OBSERVATIONAL STUDYOBSERVATIONAL STUDY

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Experimental orExperimental orintervention studyintervention study

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Experimental study Experimental study

• Put afford to control the situation of study factors and observe effect of them

• Control extraneous factors by holding those factors fix or “randomization” to make two groups have the same distribution of extraneous factors.

• One group of subject is given experimental treatment and another group (control) is given either none or less preferable. After a period effect of exposure is measured and compared between two groups.

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• RandomizationRandomization is used to allocate subject: subject get treatment by chance

• One important method to make the result more valid is called ““double double blindingblinding””:: neither investigator nor subjects know the treatment to which they have been allocated

Experimental study Experimental study

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Limitation:Limitation:• Effect is observed in artificial setting because the controlling extraneous factors

• Randomization may not be ethical

• Apart from pharmaceutical intervention, other intervention is difficult to do double blinding

Experimental studyExperimental study

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Study design

Observational Experimental

Descriptive

Case and serial case

Cross-sectional

Ecological

Analytic

Case-control

Cohort

Quasi-experimental

Experimental

Cross-sectional

Ecological

Summary of the study designSummary of the study design

Thank you, Khob Khun ka

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ECOLOGICAL STUDY

Exploratory studyExploratory study:: if there is no specific exposure of interest or the exposure of potential interest is not measureAnalytic studyAnalytic study:: if the primary exposure variable is measured and included in the analysis

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Main feature of each type of epidemiologic research

Experimental : Experimental : artificial artificial manipulation of study factormanipulation of study factorwith with randomizationrandomization

QuasiQuasi--experimental study : experimental study : artificial manipulation of the study factor artificial manipulation of the study factor without randomizationwithout randomization

Observational Observational ::no no artificial manipulationartificial manipulation of the study of the study factor factor