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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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The Framingham StudyThe Framingham Study

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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