Cohort Studies

Cohort studies outline• Big picture• Basic approach• History

– Origins– Landmark studies– Current era

• Strengths and challenges• Example: asbestos and mortality – occupational non-

concurrent, prospective cohort study• Example: estrogen and CHD – NHS concurrent,

prospective cohort study• Use of Poisson regression in cohort studies• Summary

Big picture• Fundamental study design in observational

epidemiology• Other observational designs are best understood

as sampling from a cohort experience– Some do this literally– Others theoretically

Basic approach• Group of people identified and followed up

over time to ascertain health outcomes– A cohort study could involve one population group or

cohort• Example: population representative cohort of the NYC

metropolitan area followed over the 2 years after 9/11/2001

Basic approach– A cohort study could involve two or more cohorts

(sometimes called a double cohort)• Example: exposed cohort of premature infants who had

received palivizumab (anti-RSV monoclonal antibody) in the first year of life compared to unexposed cohort of premature infants (no palivizumab)

– All called cohort studies

Basic approach

Szklo Figure 1-13JC: dots are incident cases; arrows are losses to follow-up

TimeIncident disease

Lost to Follow-up

Cohort Study

Basic approach• Illustration of collection and analysis of data from

a cohort

Szklo Figure 1-14

Basic approach• Cohort studies can be conducted looking back in

time, forward in time, or some combination

Szklo Figure 1-16

Basic approach• Terminology on types/characteristics of cohort

studies for this class– Investigator perspective

• Concurrent: investigator begins study before outcome occurs• Non-concurrent: investigator begins study after outcome occurs

Basic approach• Terminology cont.• SER teaching epidemiology symposium – Tim Lash

– Timing of record of exposure with respect to outcome• Prospective: exposure data recorded before outcome occurs (can

be recorded in records that investigator later abstract, by interview before outcome occurs etc.)

• Retrospective: exposure data recorded after outcome occurs• Understand how and when data collection took place and consider

the potential for bias in that process

Basic approach• Group identified by particular exposures,

particular population, occupational group, convenience (depends on purpose of study)– Will describe some examples in detail after covering

the basics

Basic approach• One strong advantage of a cohort study over

other designs is that the dynamic nature of many exposures and their relations in time to disease occurrence is most comprehensively captured in the cohort design

Basic approach• However, classification of exposure over time

may be extremely complicated– Participants may have different exposures for different

time periods during the study– Effects of exposure may be expected to accumulate– Participants with similar average or cumulative past

exposures may have different patterns of exposure that do not have the same effect on disease

• Example: heavy smoker in youth, light smoker currently vs light smoker in youth, heavy smoker currently

Basic approach– Time between when an exposure would have caused

disease and when disease was detected must be considered

– There may be little information available to guide decisions about how to classify exposure or what latency period is appropriate

Basic approach• Outcomes are ascertained over follow-up time

– Outcome examples: death, disease incidence (single or multiple occurrence), change in a measure of health status (e.g., BMI, depression symptoms)

– Outcomes must be common enough in the population studied or the design will be inefficient

Basic approach– Example: Leukemia incidence is approximately

10/100,000 person-years in the general population; a massive cohort would be required to detect a reasonable number of leukemia cases over a reasonable follow-up period

Basic approach• Possible data sources• Exposure

– Records• Occupational• Medical/pharmacy

– Interviews/questionnaires– Direct measurements on participants

• Outcome– Vital records– Disease registries– Medical records– Interviews/questionnaires– Direct measurements on participants

Basic approach• Closed or fixed cohort study collects information

from a group defined at the start of follow-up– Szklo Fig 1-13– Example: study of the effects of exposure to hurricane

Katrina on mental health of children with recruitment from schools over a short enrollment period and follow-up over several years

Basic approach• Open cohort study collects information from a

changing roster of individuals– Example: study of effects of HAART on progression of

HIV disease with participants identified when they first test positive for HIV with enrollment continuing over 10 years

Basic approach• Open cohort example

Potterat et al. 2004