Lesson 10: Cohort studies

What type of studies are cohort studies?

They are observational, analytical and longitudinal studies

What is the objective of a cohort study?

To follow up one or more groups of healthy people along time, and to measure the appearance of a disease or study condition.

What is the usefulness of cohort studies?

Causality investigation to test hypotheses

What is the initial requirement for subjects in a cohort study?

All subjects in the study population are free of the studied disease at the beginning.

How are subjects classified?

As exposed and non-exposed.

What happens during follow-up?

Subjects are followed over time to identify new cases of the disease.

What measures can be estimated in both groups?

Incidence and/or incidence rates.

What is the measure of association in cohort studies?

The Relative Risk (RR).

What are cohort studies used to assess?

Whether a given exposure is associated with a disease.

What are the three axes of analytical cohort studies?

Directionality of the study, follow-up, and tracking time.

What is a prospective cohort study?,

Forward in time, from exposure to outcome (disease).

What is a retrospective cohort study?

Exposure and outcome occurred earlier; historical or retrospective cohort.

What is a mixed or ambispective study?

Exposure occurred in the past and the outcome is detected now.

How are individuals selected in a cohort study?

By exposure (exposed sample), with a non-exposed comparison sample selected from the same population base.

How is tracking time measured?

In years or months.

What is a closed or static cohort?

A cohort with a fixed number of individuals at the beginning and throughout the follow-up.

What is an open or dynamic cohort?

A cohort where individuals continuously come in and out.

What measure is used in open or dynamic cohorts?

The incidence rate (incidence density), using the term person-years.

What are the uses of cohort studies?,

-Estimate incidence of disease in exposed and non-exposed individuals;

-estimate the risk of disease in the exposed relative to the non-exposed (RR);

-estimate existence of a dose-response relation;

-study the natural history of a disease;

-study more than one health consequence of exposures."

What are the advantages of cohort studies?

-Calculation of incidences and obtaining the RR

-reduced possibility of bias

-allow study of infrequent exposures

-allows study of associations between risk factors and disease occurrence

-have a higher degree of scientific evidence."

What are the disadvantages of cohort studies?

-Only a small number of risk factors can be studied

-loss of patient follow-up

-high cost in time and money

-chronic processes require long-duration studies

-low frequency diseases may need very large sample sizes; difficult reproducibility

-complex and analytical

What does RR = 1 mean?

There is no association; incidence rate is the same in exposed and non-exposed groups.

What does RR > 1 indicate?

Higher or increased risk for the exposed group; positive association.

What does RR < 1 indicate?

Lower or reduced risk for the exposed group (protective factor); negative or beneficial association.

What do nested case-control studies allow?

Identification of a pool of diseased subjects at the end of the follow-up period within a cohort.

How do cohort and case-control studies differ in group selection?,

Cohort: selection of groups by exposure; Case-control: selection of groups by disease."

Differences of case control and cohort