Module 8 Flashcards
Cohort definition
- A group of persons being studied who were born in a the same year, generation, or time period.
- Persons placed in a group who are studied as a group, over time.
Are cohort studies descriptive, analytic, or both?
Analytic- tests a specific “a priori” hypothesis often developed in descriptive studies.
Characteristics of cohort studies
Has a comparison group
Examines associations between exposure (s) and outcome(s)
Start with a group of participants who lack a positive hx of the outcome of interest and are at risk for the outcome.
Include at least two observation points:
1. One to determine eligibility and exposure status
2. A second or more to determine the number of incident cases
What is the chief characteristic of a cohort study?
It begins with ppl with the exposure and compares them to ppl without the exposure (comparison) group over time in respect to incidence or mortality of the dz
How to determine source population for cohort study
Pop of interest should be reviewed to ascertain those ppl or groups that are susceptible to becoming cases
Individuals who already have a disease outcome of interest (prevalent cases) or who are not at risk should be excluded from the study.
What are the two main types of samples of a cohort study?
Population-based
Exposure-based
Characteristics of population-based cohort studies
The cohort includes either an entire population or a representative sample of the pop
Because representative of a pop, generalizability is often good in this type of study.
Exposures are unknown until the first period of observation where exposure information is collected
Because participants were not chosen based on their exposure status, many exposures can be studied.
Definition of exposure-based cohort
Made up of participants with a common exposure. They are recruited because of this exposure
Constructing an exposure-based cohort study
Samples are taken from each of the two pops:
1. Those who have the exposure
2. Those who don’t- a group similar in demographics and geography to the exposed group, but lacks the exposure
The two cohorts are followed up and the outcome of interest measured later in time
Aspects of exposure-based cohort studies
These studies overcome a limitation of pop-based cohort studies, which are not efficient for rare exposures
Exposure-based cohort studies are best when the exposure is rare and a relatively small number of ppl are affected.
Most often used in environmental or occupational epidemiology.
How comparison is done to the unexposed group
Several categories of exposure may exist, and the lowest is considered “unexposed”
In some studies, exposed groups are compared to the general pop, which is then considered the “unexposed” group.
What are three classifications of cohort studies according to time of data collection in relation to when the study was planned?
Prospective
Retrospective
Ambispective (a mixture of prospective and retrospective)
Definition and characteristics of prospective study
The study is planned and the exposure is measured before the outcome has occurred.
Wait for outcome to occur
Purely prospective in nature; characterized by determination of exposure levels at baseline and f/u for occurrence of dz at some time in the future.
Advantages of prospective cohort studies
Enable the investigator to collect primary data on exposures; the most direct and specific test of the study hypothesis
Direct measures of the environment can be made
The size of the cohort is under greater control of the investigators
Biological and physical assays can be performed with decreased concern that the results will be affected by the underlying dz process.
Disadvantages of prospective cohort studies
Despite substantial benefits of prospective cohort studies, investigators have to wait for cases to accrue.
High cost in time
Very expensive. Must be monitored for years, decades.
