searo – csr training on outbreak investigation cohort and case-control studies observational...

SEARO – CSR Training on Outbreak Investigation

Cohort and case-control studies

Observational studies

Cohort and case-control studies SEARO – CSR Training on Outbreak Investigation

Learning Objectives

• At the end of the sessions, the participants will be able to:– Define cohort and case-control studies– Identify applications for cohort and case-

control studies– Describe two-by-two table– Compute different measures of association


Cohort study

Observational studies: Cohort and case-control studies


Learning Objectives

• At the end of the session, the participants will be able to:– Define cohort study– Describe prospective and retrospective

cohort studies– Compute attack rate and relative risk– Enumerate pros and cons in using cohort

studies


What is a cohort?

• Cohort: Latin word for one of the 10 divisions of a Roman legion

• A group of individuals – Sharing same experience – Followed-up for a specified period of time

• Examples– Birth cohort– Occupational cohort chemical plant workers– A Rapid Response Team


Exposureoccurrence

Study starts Diseaseoccurrence

Growth-nutrition studies, Folic acid and NT defects

Prospective Cohort Study

Time

+

-

+ -ill

exp+

-exp

Prospective assessment of exposure and disease

Selection of population


Prospective cohort study

Chernobyl, Industrial accidents, Flood victims

+

-

+ -ill

exp+

-exp

Diseaseoccurrence

Study startsExposureoccurrence

Prospective assessment of disease

Selection based on exposure


Food borne outbreaks, closed environment outbreaks (school, prisons, etc)

Retrospective cohort study Transversal studies

Study takes place

Diseaseoccurrence

Exposureoccurrence

Retrospective assessment of exposure and disease

Selection based on population

+

-

+ -ill

exp

Real TimeNow


Presentation of Cohort Data two-by-two table

Exposed

Unexposed

ill not ill

a

c

b

d

Incidence in exposed (Ie+) = a/(a+b) Incidence in non-exposed (Ie-) = c/(c+d)


• Hypothesis– Is the incidence among exposed higher than among unexposed

• Absolute measures

– Risk difference (RD) Ie+ - Ie-

• Relative measures– Relative risk (RR)

Rate ratio Risk ratio

Effect measures in cohort studies

Ie+

Ie-

a/(a+b)c/(c+d)

=


Ate ham

Did not eat ham

ill not ill

Risk difference 0.5 - 0.4 = 0.1 (10%)

Relative risk 0.5 / 0.4 = 1.25

Foodborne Outbreak in a Wedding, Dublin

Incidence

49 49 98 50%

4 6 10 40%

53 55 108


Does HIV infection increase the risk of developing TB among a population of drug users?

Source: Selwyn et al., New York, 1989

Presentation of cohort data Population at risk

Drug users (f/u 2 years)

8

HIV -

TB Cases

HIV + 215

289 1

Incidence(%)

Relativerisk

3.7 (8/215)12

0.3 (1/289)


Advantages and disadvantages of cohort studies

• Advantages– Can measure

incidence and risks– Good for rare

exposures– Clear temporal

relationship between exposure and outcome

– Less subject to selection bias

• Disadvantages– Requires a large

sample size– Latency period– Lost to follow-up– Ethical considerations– Resource intensive

• High cost• Timely


Case-Control study

Observational studies: Cohort and case-control studies


Learning Objectives

• At the end of the session, the participants will be able to:– Define case-control study– Describe steps in setting up a case-control

study– Compute Odds Ratio– Enumerate pros and cons in using case-

control studies


Retrospective assessment of exposure

+ -ill

Diseaseoccurred

Exposureoccurred

Study takes place

+

-

+ -ill

exp

Case-Control Study

Selection based on disease status

Real TimeNow


Setting-up a case-control study

• Identify group of cases• Identify group of controls• Question both groups for possible

exposure• Measure the frequency of exposure

occurrence in both groups• Compare the frequency of exposure

between cases and controls


Presentation of case-control data two-by-two table

Exposed

Not exposed

case control

a

c

b

d

% of exposure among cases = a/(a+c) % of exposure among controls = b/(b+d)


Odds of exposure

Exposed

Not exposed

cases controls

18

19

14

43

37 57


Effect measures in case-control studies

OR =a/cb/d

a*db*c

=

• OR = 1 – no association

• OR > 1 – there is an association

• OR < 1 – the factor is some way protective


Drink waterFrom well Cholera Control

Yes 30 25

No 32 60

Total 62 85

% exposed 48.4% 29.4 %

30 * 60 OR = ----------- = 2.25 (95%CI 1.07-4.75)

32 * 25

Distribution of Cholera Cases and Controls by Exposure to Shallow Well WaterN'Djamena, July 2001


Multiple Exposure Levels

B1High A1

DNot exposed C

CasesExposurelevel

B2Medium A2

B3Low A3

OR1

OR2

OR3

Reference

Controls OR


Relation of Hepato cellular Adenoma to duration of oralcontraceptive use in 79 cases and 220 controls

Months ofOC use Cases Controls Odds ratio

0-12 7 121 Ref.13-36 11 49 3.937-60 20 23 15.061-84 21 20 18.1>= 85 20 7 49.7Total 79 220

Source: Rooks & col. 1979


Advantages and disadvantages of case-control studies

• Advantages– Suitable for rare

diseases– Can explore several

exposures– Low cost– Rapid – Can cope with long

latency– Small sample size– No ethical problems

• Disadvantages– Cannot calculate the

risk– Not suitable for rare

exposures– Temporal

relationship difficult to establish

– Subject to bias• Selection of controls• Recall bias• …


• Cohort studies are better but harder to carry out and provide true measure of risk

• Case-control studies are rapid and easy to carry out, but only provide estimates of risks

• Prefer cohort to case-control when feasible

• In field epidemiology, case-control studies are more frequently used

Conclusions

searo – csr training on outbreak investigation cohort and case-control studies observational...

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