Transcript chap7-eks-3ed.ppt

Chapter 7: Observational
Cohort Studies
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Ch 7: Cohort Studies
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Ch 7 Outline
7.1 Introduction
7.2 Historical perspective
7.3 Assembling and following a cohort
7.4 Prospective, retrospective, and ambidirectional
7.5 Addressing the potential for confounding
7.6 Data analysis
7.7 Wade Hampton Frost (optional)
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Basic Cohort Design
• Recruit study subjects
• Classify individual as exposed or non-exposed
• Follow individuals over time, ascertain outcomes
• Compare incidences of study outcomes
Closed
population
Exposed
individuals
non-exposed
individuals
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Incidence1
RR or RD
Incidence0
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Historical Perspective
• From the Latin cohors
– “an enclosure”
– a unit of the Roman army
• Historical notes
• Ramazzini studied worker health
(1713)
• Louis studied clinical outcomes in
patients (18th century)
• Pinel studied mental health
outcomes associated with humane
treatment (18th century)
• Doll & Hill studied British doctors
and smokers (1951 - present)
• Americans studied the causes of
heart disease in Framingham
Massachusetts
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Do not lose sight of
individual experiences
from the beginning to
the end (William Farr)
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British Doctors Study Survival Curves
Begun in 1951 by Doll and Hill with a mailing of a 6 question
questionnaire sent to 59,600 individual addresses.
80% of nonsmoker
survived to age 70
50% heavy smokers
survived to 70
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.
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WHI Observational Cohort
• WHI project included
both experimental and
observational cohorts
• Observational study:
93,676 women, 50-79
@ 40 clinical centers
• Recruitment period
1993 - 1998
• Up to 15 years of
follow-up
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WHI Observational Cohort
Breast CA & Analgesic Use
Group
Duration
Of use
Referent
< 1 yr
n
BreastCA
cases
PYs
Inc. Rate
per 1000
P-value
trend
54,102
955
194,884
49.0
N/A
NSAIDs
1–4 yr
≥5 yr
9,000
10,162
148
83
32,127
36,576
46.4
40.5
0.01
Aspirin
1–4 yr
≥5 yr
5,124
6,759
149
99
18,231
24,398
45.5
40.6
0.03
Acetamin.
1–4 yr
≥5 yr
2,450
4,675
44
79
8,608
16,698
51.1
47.3
0.71
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Prospective, Retrospective,
Ambidirectional
•
•
•
•
Based on proximity of data collection to actual events
Prospective = data collected near time of event
Retrospective = data are from the past (“historical data”)
Ambidirectional - combination of prospective and
retrospective data
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WHI analgesic / breast cancer study
Proximity of Data Collection
Initial data collection
Analgesic use history Breast cancer occurrence
The ascertainment of the exposure was retrospective.
The ascertainment of the outcome (breast cancer) was
prospective.
Overall, the study was ambi-directional.
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Example 7.4 (Retrospective Cohort)
Chemical workers and bladder cancer
• Exposure to benzidine, β-naphthylamine, alpha-naphthylamine
• Historical cohort: occupational records of 4622 men in the British
chemical industry, 1900 to 1950
• Bladder cancer info from death certificates 1921 – 1950
• Data collection: 1952
• Retrospective (“historical”) cohort study
Initial data collection
Chemical Exposures Bladder tumors
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Multiple levels of exposure
Analgesics and Breast cancer
Group
Duration
Of use
Referent
< 1 yr
NSAIDs
•
•
•
•
•
•
•
1–4 yr
≥5 yr
n
BreastCA
cases
PYs
Rate per
1000
54,102
955
194,884
4.90
9,000
10,162
148
83
32,127
36,576
4.64
4.5
P-value
trend
N/A
0.01
Let R0 ≡ rate per 1000 p-yrs in referent group
Let R1 ≡ rate per 1000 p-yrs in NSAID users of 1-4 years
Let R2 ≡ rate per 1000 p-yrs in NSAID users of 5+ years
RR1 = R1 / R0 = 46.4 / 49.0 = 0.95 [no units]
RR2 = R2 / R0 = 40.5 / 49.0 = 0.83 [no units]
RD1 = R1 − R0 = 46.4 − 49.0 = −2.6 per 1000 p-yrs
RD2 = R2 − R0 = 40.5 − 49.0 = −8.5 per 1000 p-yrs
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OpenEpi.com for data analysis
• “Counts” menu for
incidence proportions,
prevalences, and casecontrol data
• “Person Time” menu for
rate data
• Descriptive and inferential
(confidence intervals and
P-values) statistics
• Can be used as a
learning tool
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