(c) Gerstman Ch 11: Observational Designs 1

Epidemiology Kept Simple

Ch 11: Observational Studies

(c) Gerstman Ch 11: Observational Designs 2

Observational Designs • Cross-sectional: Sample population, no

follow-up of individuals compare disease experience of exposure groups (§11.2 and §11.3)

• Cohort: closed population with individual follow-up over time compare disease experience of exposure groups (§11.4)

• Case-control: all cases and a sample of non-cases from population compare exposure experience (§11.5)

(c) Gerstman Ch 11: Observational Designs 3

Cross-Sectional Designs• Recall distinction between longitudinal and

cross-sectional observations

• Recall the distinction between individual and aggregate units of observation

• Cross-sectional design with aggregate unit of observation ≡ ecological design

• Cross-sectional data with individual units ≡ cross-sectional survey

(c) Gerstman Ch 11: Observational Designs 4

Example: Ecological DataUnit of observation = geographic region

Exposure = Cig1930 = cigarettes per capita, 1930

Disease = Mortal = lung cancer mortality per 100,000 p-yrs, 1950

(c) Gerstman Ch 11: Observational Designs 5

r = 0.74

Example: Ecological Data

(c) Gerstman Ch 11: Observational Designs 9

Example: X-Sectional Survey

SES & Mental Disorders Prevalence per 100,000

Social class Psychosis Neurosis

High 188 349

Moderate 291 250

Low 518 114

Very low 1505 97

(c) Gerstman Ch 11: Observational Designs 11

§11.4 Cohort Studies

Closed populatio

n

Incidence1

Incidence0

RR or RD

•Recruit cohort •Classify individual as exposed or non-exposed•Follow exposed and non-exposed sub-cohorts to determine incidence

exposedsub-cohort

non-exposedsub-cohort

RR or RD

RR or RD

RR or RD

(c) Gerstman Ch 11: Observational Designs 14

British Doctors Cohort

Source: Doll, R., Peto, R., Wheatley, K., Gray, R., & Sutherland, I. (1994). Mortality in relation to smoking: 40 years' observations on male British doctors. British Medical Journal, 309(6959), 901-911.

80% of nonsmoker survived to age 70

50% heavy smokers survived to 70

(c) Gerstman Ch 11: Observational Designs 22

Example: Historical Cohort• Historical info on exposure to

aniline dyes (from work records) were used to compile exposed and non-exposed worker cohorts

• Retrospective data from death certificates on bladder cancer occurrence

• Result: bladder cancer occurrence was 100 times as frequent in aniline-exposed cohort

• Figure shows induction time between exposure onset and bladder CA occurrence

(c) Gerstman Ch 11: Observational Designs 23

Case-Control Studies• Identify population cases• Randomly select non-cases (“controls”)• Compare exposure histories in cases & controls

Population

All cases

Sample non-cases

Exposure histories

Exposure histories

Odds Ratio

(c) Gerstman Ch 11: Observational Designs 24

Case-Control

Cases Controls

Exposed A1 B1

Non- exposed A0 B0

M1 M2

01

01Ratio OddsAB

BA

Cross-tabulate disease and exposure statusof cases and controls

Calculate:

(c) Gerstman Ch 11: Observational Designs 26

Interpretation of the Odds Ratio

• When the disease is rare, interpret the OR as if it were an RR

• The illustrative OR of 9.3 suggests that tampon users had 9.3 times the risk as non-tampon users

• [The suspected brand of tampon has since been removed from the market]

(c) Gerstman Ch 11: Observational Designs 27

Multiple Levels of ExposureHistorical Example: (Wynder & Graham, 1950, p. 212)

Smoking status Cases Controls

Chain 123 64

Excessive 186 98

Heavy 213 274

Moderate 61 147

Light 14 82

Non-smoker 8 115

Total 605 780Table 1 Case Cntl

Light smoke 14 82

Non-smoker 8 115

OR1= (14)(115)/(82)(8) = 2.5

Table 2 Case Cntl

Moderate 61 148

Non-smoker 8 115

OR2=(61)(115)/(147)(8) = 6.0

Table 3 Case Cntl

Heavy 213 274

Non-sm. 8 115

OR3= (213)(115)/(274)(8) =11.2

Exposure may be measured at various levels. In this historical example, smoking is classified into 6 levels. To analyze the table, break-up it up into five separate 2-by-2 tables with each table referencing the nonexposed group as follows:

Table 4 Case Cntl

Excessive 186 98

Non-smoker 8 115

OR4 = (186)(115)/(98)(8) = 27.3

Table 5 Case Cntl

Chain 123 64

Non-smoker 8 115

OR5= (123)(115)/(64)(8) =27.6

(c) Gerstman Ch 11: Observational Designs 28

Longitudinal Designs

(c) Gerstman Ch 11: Observational Designs 29

Longitudinal Designs

Trial Cohort Case-ControlExperimental Observational Observational

Assign exposure Classify exposure Select cases and non-cases

Calculate RD and/or RR

Calculate RD and/or RR

Calculate OR

Study multiple outcomes

Study multiple outcomes

Study multiple exposures

Prospective Prospective or retrospective

Retrospective

(c) Gerstman Ch 11: Observational Designs 30

End of HS 261 Presentation