Cohort Studies

• All studies involve some descriptive or analytic type of comparison of exposure and disease status.

• Analytical study design options include: observational or interventional (which one is based on the role of the investigator).

• There are three basic types of observational analytical study designs:

Cohort studies Case-control studies Cross-sectional studies

Introduction

• Specific epidemiological study designs can be used to reveal etiologic (causal) relationships

• First, using observational analytical studies :Determine whether there is an association

between a factor or a characteristic and the development of disease

• Second:From these associations, derive appropriate

inferences regarding a possible causal relationship

Introduction

Analytical Studies

• Control and experimental groups

• Randomized groups data collected without bias

• Dependent and independent factors

• Group by common characteristics

• Start with a group of subjects who lack a positive history of the outcome of interest yet are at risk for it (cohort). Think of going from cause to effect.

• The exposure of interest is determined for each member of the cohort and the group is followed to document incidence in the exposed and non-exposed members.

Cohort Studies

When is a cohort study warranted?

• When good evidence suggests an association of a disease with a certain exposure or exposures.

• Changes and variation in the disease or health status of a study population as the study group moves through time.

• “Generation effect”

Cohort Effect

• Prospective (concurrent)

• Retrospective (historical)

• Restricted (restricted exposures)

Types of Cohort Studies

Types of Cohort Studies• Prospective – cohort characterized by

determination of exposure levels (exposed vs. not exposed) at baseline (present) and followed for occurrence of disease in future Groups move through time as they age

• Retrospective - makes use of historical data to determine exposure level at some baseline in the past and then determine subsequent disease status in the present.

• Restricted - limited exposure, narrow behavior (military, long shore men)

Prospective Studies

• Also called longitudinal concurrent incidence studies

• Looking into the future

• Example: Framingham Study of coronary heart

disease (CHD)

• The essential characteristic in the design of cohort studies is the comparison of outcome in an exposed group and a nonexposed group (or a group with a certain characteristic and a group w/o that characteristic).

A study population can be chosen by selecting groups for inclusion in the study on the basis of whether or not they were exposed

Design of a Cohort Experiment

• There are two basic ways to generate cohort groups. Select a cohort (defined population) BEFORE

any of its members become exposed or before the exposures are identified.

Select a cohort on the basis of some factor (e.g., where they live) and take histories (e.g., blood tests) on the entire population to separate into exposed and non-exposed groups.

• Regardless of which selection approach is used, we are comparing exposed and non-exposed persons.

Selection of Cohort Groups

Design of a Cohort Experiment

Design of a Prospective Cohort Experiment

Major problem with a prospective cohort design is that the cohort must be followed up for a long period of time.

Framingham Study

• Designed to study the effect of multiple factors on coronary heart disease (CHD):agehypertensionelevated blood cholesteroltobacco smokingincreased physical activityincrease in body weightdiabetes mellitus

• Framingham, Massachusetts population was 28,000

• Study design called for a random sample of 6,500

• Enrollment questionnaire form targeted age range 30-59 years

• No clinical evidence of atherosclerotic cardiovascular disease

• Cohort re-examined every two years

• Problems: white, middle class

Framingham Study Design

Hypothetical Cohort Study Approach

1. Examine people over their life times for contraction of diseases. In meantime, ask questions concerning diet, lifestyle,habits, work, etc.

2. Take blood and do lab tests (as many as possible).

3. Do same tests every year for a period of years.

Interventional aspect: If positive findings occur, they refer you to a specialist. By screening, they may have prolonged a life.

Sampling• Valid, reliable surveys • Critical number of subjects

the more, the better

• Randomize random selection random assignment

• Rule out bias For example, degree of accuracy with which

subjects have been classified with respect to their exposure.

For example, individuals who are sick may be more likely to give the kind of responses that they believe the investigator wants to hear

Garbage in,garbage out

Data Gathering

• Person - to - person

• Drop off questionnaire

• Mailed to people

• Telephone interview

• Newsletter or magazine

Potential Biases in Cohort Studies

• Information bias

• Bias in estimation of the outcome

• Bias from non-response

• Bias from losses to follow-up

• Analytic biasYour assignment:

Describe and differentiate between these types of biases.

Advantages of Prospective Cohort Studies

• Captive groups• Large sample sizes• Certain diseases or risk factors targeted• Can be used to prove cause-effect• Assess magnitude of risk• Baseline of rates• Number and proportion of cases that

can be prevented

Advantages of Prospective Studies (cont’d)

• Completeness and accuracy

• Opportunity to avoid condition being studied

• Quality of data is high

• Considers seasonal and other variations over a long period

• Tracks effects of aging process

Disadvantages of Prospective Cohort Studies

• Large study populations required not easy to find subjects

• Expensive

• Unpredictable variables

• Results not extrapolated to general population

• Study results are limited

• Time consuming/results are delayed

• Requires rigid design and conditions

Disadvantages of Prospective Studies (cont’d)

• Subjects lost over time (dropouts)

• Costs are high

• Logistically demanding

• Maintaining quality, validity, accuracy and reliability can be a problem

Survivorship Studies• Survivorship is the number of persons out of

a study population who would survive until a certain time interval has been reached

• Shows the chance that an event (such as death from cancer) will occur in successive intervals of time once a diagnosis has been made

• Analysis yields a cumulative probability of surviving the projected time periodFor infectious diseases, we use case fatality rate

to assess survival For chronic diseases, we use cohort life tables

(Cohort) Life Tables

• Charts which summarize the patterns of survival and death in study groups of certain types of disease (chronic)

• Insurance companies study these charts very closely.

Dip at beginning of life is due to infant mortality rate.

As one reaches the

later years of life, the survival curve goes down and the risk of death goes up.

Survival curves and risk of death for males vs. females based on life tables in California for 1980.

Is the Association Causal?

To be continued…