Chapter 8- Cohort studies

Question 1

Cohort study

Answer 1

An observational study where the investigator selects a group of exposed individuals and a group of unexposed individuals, and follows both groups over time to compare the incidence of disease or rate of death from disease in both groups. These studies have to wait for the outcome to occur, which may require a long period of follow up

Question 2

Positive association

Answer 2

If a positive association exists between the exposure and the disease, we would expect that the proportion of the exposed group where the disease develops would be greater than the proportion of the unexposed group where the disease develops. If we are identifying new (incident) cases of disease as they occur, we can determine whether a temporal relationship exists between the exposure and the disease- where exposure preceded the onset of disease

Question 3

Limitations/considerations of cohort studies (4)

Answer 3

1. How to select a study population?
2. Types of cohort studies
3. Selection biases
4. Information biases

Question 4

Selection of study populations

Answer 4

You can identify participants based on known exposure or can recruit a defined population without regard to exposure

Question 5

Types of cohort studies (2)

Answer 5

1. Prospective (longitudinal, concurrent)
2. Retrospective (historical, nonconcurrent)

Question 6

Prospective cohort studies

Answer 6

The investigator identifies the original population at the beginning of the study and follows the subjects concurrently through a period of time, until the disease either develops or does not develop. Exposure and unexposure are determined as they occur during the study. This type of study can take decades and can be expensive

Question 7

Retrospective cohort studies

Answer 7

The study design does not differ from the prospective study. A retrospective study uses historical data in order to shorten the time period of the study. Exposure is ascertained from past records and the outcome is determined when the study begins

Question 8

Framingham Heart Study

Answer 8

A well known cohort study in Massachusetts. Residents of Framingham could participate if they were 30-62 so they would be more likely to develop cardiovascular disease during the study period. The study population was examined every 2 years for incidence of new coronary events. The study was designed to test whether the incidence of CHD increases with age and whether it increases due to various exposures. This study was advantageous because it allowed investigators to study multiple exposures (hypertension, smoking, obesity, cholesterol levels) and complex interactions between the exposures.

Question 9

Potential biases in cohort studies (2)

Answer 9

1. Selection bias
2. Information bias

Question 10

Selection bias

Answer 10

Occurs when selection or participation is related to exposure AND outcome. Participants who refuse to join a study may skew the results if the participants who refuse to join are also more likely to have a specific exposure. Also, participants who are lost to follow up may impact the results.

Question 11

Healthy worker effect

Answer 11

A type of selection bias comparing occupationally-exposed cohorts with unexposed persons from general population

Question 12

Information bias

Answer 12

Occurs when measurement is study is related to exposure AND outcome. If the quality and extent of information obtained is different for exposed persons than for the unexposed persons, bias will be introduced. This is more likely in retrospective studies. If the person who decides whether the disease has developed in each subject knows whether the subject has been exposed and is also aware of the hypothesis being tested, it could bias their judgement of whether the illness has developed. This person should be masked. The epidemiologists analyzing the data may introduce bias if they have strong preconceptions

Question 13

Benefits of cohort studies (2)

Answer 13

1. Direct comparison of disease risk between exposed & unexposed (goal in analytic epi)
2. More practical, when relationship between exposure & outcome is short
   Cohort Studies

Question 14

If exposure is associated with disease, we would expect to find

Answer 14

A greater incidence of disease in the exposed group than in the unexposed group

Question 15

When is a cohort study indicated?

Answer 15

When good evidence suggests an association of a disease with a certain exposure

Question 16

Case control studies based within a defined cohort

Answer 16

Combines case control and cohort studies- a population is identified and followed over time. When the population is identified, baseline data is collected. When a small proportion of the participants develop the disease over time, a case control study is then carried out using people who developed the disease as cases and using a sample of people who did not develop the disease as controls. There are 2 types of these studies- nested case control studies and case-cohort studies

Question 17

2 types of Case-Control Studies within a Defined Cohort

Answer 17

1. Nested case-control design
2. Case-cohort design

Question 18

Nested case-control studies

Answer 18

The controls are a sample of individuals who are at risk for the disease at the time each case of the disease develops. At the time each case develops, the same number of controls is selected. A control who is selected early in the study could develop the disease and later become a case in the study

Question 19

Case-cohort studies

Answer 19

Cases develop at the same times that were seen in the nested case-control design, but the controls are randomly chosen from the defined cohort with which the study began. The subset of the full cohort is called the sub cohort.

Question 20

Advantage of case-cohort studies

Answer 20

Because controls are not individually matched to each case, it’s possible to study different diseases (different sets of cases) in the same case-cohort study using the same cohort for controls

Question 21

Benefits of Case-Control Studies within a Defined Cohort (4)

Answer 21

Combines advantages of cohort and case-control studies
1. Better establishes temporality
2. Limits information bias [differential recall of exposure]
3. Less expensive than cohort study
4. Greater comparability between cases & controls because BOTH are ascertained from same original cohort