(c) gerstmanch 11: observational designs1 epidemiology kept simple ch 11: observational studies
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(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