Secondary Study Base
Lead Editor(s): Jeff Martin, MD
A study base, also called a reference population, is a defined population whose disease experience during some period of time is the source of the study data.
Secondary Study Base
Secondary Study Base = population that gave rise to cases -
- that is, those persons who would have been cases if they had disease diagnosed during the time period of study.
- Start with cases and then attempt to identify hypothetical cohort that gave rise to them.
- Difficult concept but crucial to Case-control design outside a well defined population.
The concept of a secondary study base occurs because cases of a given disease may be identified, but they do not come from a clearly defined population such as a cohort, geographic area, or HMO. In other words, you have the cases first and you need to determine what study base gave rise to them.
- Typically these are cases of a disease seen in single hospital or other health care facility (or a limited number of hospitals/facilities). They may appear to come from a geographic area since hospitals that are not referral centers draw most of their patients from persons living in their geographical vicinity, but the difference is that the boundaries of the geographic area are difficult to determine, and there is no guarantee that many of the cases from the hospital’s catchment area are not being seen at other hospitals.
Think of taking the cases of a given disease in one San Francisco hospital and trying to decide what geographic area they represent. All of the patient addresses for persons with diagnosis during some time period could be mapped and a boundary drawn around them, but many other cases not seen at the study hospital were probably diagnosed within that boundary and seen at other hospitals. Without nearly complete case ascertainment, there is no way to know how the characteristics of the patients who chose to come to the study hospital differ from those who went elsewhere.
For the controls to come from the same study base as the cases, they need to be those persons who would come to the study hospital if they did have the disease of interest.
Case Control Studies from a Secondary Study Base
(1) Source of cases is often one or more hospitals or other medical facilities
(2) Problem is identifying who would come to the facility if diagnosed with the disease
(3) Careful consideration has to be given to factors causing someone to show up at that institution with that diagnosis
Since trying to identify a geographic catchment area to define the study base population does not work in most instances, it is necessary to give careful thought to what characteristics of the cases are causing them to show up at the study hospital.
- If those characteristics are associated with the study's main predictor variable, substantial bias can occur in the measure of association by choosing inappropriate controls.
Finding Secondary Study Base: Glioma Cases Seen at UCSF
An example of finding a secondary study base comes from glioma cases seen at UCSF.
- Difficult because referrals come from many areas
- Since UCSF is a major referral center from brain tumors, it is very difficult to determine the secondary study base for glioma cases.
Control Group: UCSF Patients
One possible control group might be UCSF patients with a different neurologic disease
- If patients with a different neurologic disease for which UCSF is also a referral center are patients who would also have come to UCSF had they been diagnosed with a glioma instead, they may represent a sample of the study base population that gave rise to the gliomas. But are all referral populations the same?
Control Group: Similiar Tertiary Clinic
Another possible control group might be patients from a similar tertiary referral clinic.
Control Group: Neighborhood
Another possible control group might be residents of the neighborhood of the case are another possibility
- Another approach that has been used often for hospital case-control studies is to select neighborhood controls for the cases. This approach uses geography to identify possible controls, but it chooses a very narrow boundary for each case, such as the block across the street from the case's address from which a random address is chosen. The assumption is that someone else with a glioma in that immediate neighborhood would also have come to UCSF. This may not be valid assumption.
Problems with Secondary Study Base
Main problem with a secondary base is the definition of the base
- e.g., hospital-based case-control studies common but very difficult to determine the study base
With a secondary study base, all of the cases are available since such designs usually start with new cases arriving at a hospital.
- Determining the population to sample for controls is the challenge.
- The threats to validity are generally much greater with a secondary study base than a primary study base.