Epidemiology Kept Simple

1. Epidemiology Kept Simple Section 11.4 Cohort Studies Cohort Studies

2. Cohort Studies Cohort studies follow two or more cohorts over time. Incidences are calculated and compared. n1 exposed individuals Incidence, exposed Select Compare incidences n0 nonexposed individuals Incidence, nonexposed Cohort studies are nonexperimental studies with follow-up of individuals over time Cohort Studies

3. Cohorts (Closed Populations) 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. Incidences of events are compared in two or more groups. Cohort Studies

4. This survival curves show that 80% of the nonsmoker cohort survived to age 70. Illustrative example: British Doctor’s Study 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 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. Cohort Studies

5. British Doctor’s Study The 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: Cohort Studies

6. Historical IllustrationWade Hampton Frost’s Birth Cohorts • Wade Hampton Frost was the first Professor of epidemiology 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) Cohort Studies

7. Illustrative Example: Cross-Sectional Rates vs. Cohort Rates TB Mortality per 100,000 by Age and Year1880 Cross-Sectional Rates Shaded Reading down columns shows cross-sectional rates (NO follow-up of individuals, NOT cohorts) Cohort Studies

8. Historical Illustration: Cross-Sectional vs. Cohort TB Mortality per 100,000 by Age and YearSerial Cross-Sectional Rates Shaded Reading across rows just provides cross-sectional rates in serial fashion (NO follow-up of individuals, NOT cohorts) Cohort Studies

9. Historical Illustration: Cross-Sectional vs. Cohort TB Mortality per 100,000 by Age and Year1870s Birth Cohort Rates Shaded Reading along the diagonals mimics the experience of a birth cohort over time. The experience of the 1870 birth cohort is shaded. Cohort Studies

10. Historical Illustration: Cross-Sectional vs. Cohort Analysis of Cross-Sectional Rates • Cross-sectional rates decreases over time • Peak in young children and adults – note that the adult peak is shifting to older ages (marked by *) over time Cohort Studies

11. Historical Illustration: Cross-Sectional vs. Cohort Analysis of Birth Cohort Rates • Birth cohorts rates show decreases over time with a consistent age-relation (peak in late 20s associated with child birth) • Practical implication: squelched idea of the benefit of early exposure • Bridge between infectious disease and chronic disease epidemiology Cohort Studies

12. The Framingham Study Cohort Studies

13. Retrospective Cohorts • Cohort data can be collected prospectively (as events occur over time) or retrospectively (historical records) • Dyestuff workers and bladder cancer illustrative example (p. 208) • Employment records and vital statistic used to capture events from the past • On-the-job exposure to aniline dyes was associated with a large increase in bladder cancer Cohort Studies