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 -

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. 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.

Finding Secondary Study Base: Glioma Cases Seen at UCSF

An example of finding a secondary study base comes from glioma cases seen at UCSF.

Control Group: UCSF Patients

One possible control group might be UCSF patients with a different neurologic disease

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

Problems with Secondary Study Base

Main problem with a secondary base is the definition of the base With a secondary study base, all of the cases are available since such designs usually start with new cases arriving at a hospital.