gerstman cohort studies 1 epidemiology kept simple section 11.4 cohort studies
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Epidemiology Kept SimpleEpidemiology Kept Simple
Section 11.4 Section 11.4
Cohort StudiesCohort Studies
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Cohort StudiesCohort StudiesCohort studies follow two or more cohorts over time.
Incidences are calculated and compared.
Select
n1 exposedindividualsn0 nonexposedindividuals
Incidence, exposed
Incidence,nonexposed
Compare incidence
s
Cohort studies are nonexperimental studies with follow-up of individuals over time
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Cohorts (Closed Populations)Cohorts (Closed Populations)
Incidences of events are compared in two or more groups.
Recall from Chapter 6 that a cohort is a closed population. Individuals are followed over time. The experience of cohort members is monitored to determine onsets of new events.
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Illustrative example: Illustrative example: British Doctor’s StudyBritish Doctor’s Study
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.
This famous cohort study started in the 1950 by Doll and Hill has been followed for half a century
In contrast, 50% of the > 25 cigarette / day cohort survived to age 70
This survival curves show that 80% of the nonsmoker cohort survived to age 70.
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British Doctor’s StudyBritish Doctor’s StudyThe British Doctors study has discovered or confirmed more than 40 serious adverse health outcomes associated with smoking. Here is a table from one of its earliest publications:
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Historical IllustrationHistorical IllustrationWade Hampton Frost’s Birth CohortsWade Hampton Frost’s Birth Cohorts
Wade Hampton Frost was the Wade Hampton Frost was the first Professor of epidemiology first Professor of epidemiology and public health in U.S.and public health in U.S.
His birth cohort studies have stood the test of time in illustrating the distinction between cross-sectional and longitudinal studies
These studies also provided the bridge between acute and chronic disease epidemiology
We will slowly go through some of Frost’s analyses
Wade Hampton Frost (1880 – 1938)
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Illustrative Example: Illustrative Example: Cross-Sectional Rates vs. Cohort RatesCross-Sectional Rates vs. Cohort Rates
Reading down columns shows cross-sectional rates
(NO follow-up of individuals, NOT cohorts)
TB Mortality per 100,000 by Age and YearTB Mortality per 100,000 by Age and Year1880 Cross-Sectional Rates Shaded1880 Cross-Sectional Rates Shaded
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Historical Illustration: Cross-Historical Illustration: Cross-Sectional vs. CohortSectional vs. Cohort
Reading across rows just provides cross-sectional rates in serial fashion (NO follow-up of individuals,
NOT cohorts)
TB Mortality per 100,000 by Age and YearTB Mortality per 100,000 by Age and YearSerial Cross-Sectional Rates ShadedSerial Cross-Sectional Rates Shaded
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Historical Illustration: Cross-Sectional vs. CohortHistorical Illustration: Cross-Sectional vs. Cohort
Reading along the diagonals mimics the experience of a birth cohort over time. The experience of the 1870 birth cohort is
shaded.
TB Mortality per 100,000 by Age and YearTB Mortality per 100,000 by Age and Year1870s Birth Cohort Rates Shaded1870s Birth Cohort Rates Shaded
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Historical Illustration: Cross-Sectional vs. CohortHistorical Illustration: Cross-Sectional vs. Cohort
Cross-sectional rates decreases over time Cross-sectional rates decreases over time Peak in young children and adults – note that the adult peak is Peak in young children and adults – note that the adult peak is
shifting to older ages (marked by *) over timeshifting to older ages (marked by *) over time
Analysis of Cross-Sectional Rates
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Historical Illustration: Cross-Sectional vs. CohortHistorical Illustration: Cross-Sectional vs. Cohort
Birth cohorts rates show decreases over time with a consistent age-Birth cohorts rates show decreases over time with a consistent age-relation (peak in late 20s associated with child birth)relation (peak in late 20s associated with child birth)
Practical implication: squelched idea of the benefit of early exposurePractical implication: squelched idea of the benefit of early exposure Bridge between infectious disease and chronic disease epidemiologyBridge between infectious disease and chronic disease epidemiology
Analysis of Birth Cohort RatesAnalysis of Birth Cohort Rates
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Retrospective CohortsRetrospective Cohorts Cohort data can be collected prospectively Cohort data can be collected prospectively
(as events occur over time) or retrospectively (as events occur over time) or retrospectively (historical records)(historical records)
Dyestuff workers and bladder cancer Dyestuff workers and bladder cancer illustrative example (p. 208)illustrative example (p. 208)
Employment records and vital statistic used to Employment records and vital statistic used to capture events from the pastcapture events from the past
On-the-job exposure to aniline dyes was On-the-job exposure to aniline dyes was associated with a large increase in bladder cancerassociated with a large increase in bladder cancer