Primary Study Base

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.

Lead Editor(s): Jeff Martin, MD

Definition of Primary Study Base

Primary Study Base = population that gives rise to cases that can be defined before cases appear by a geographical area or some other identifiable entity like a health delivery system or a cohort study.

Examples of Primary Study Bases

Participants in Women's HIV Cohort Study

Residents of San Francisco during 2002

Members of the Kaiser Permanente system in the Bay Area during 2002

Military personnel stationed at California bases during 2002

A dedicated research cohort, such as the Women's HIV Study, is a type of primary study base. Case-control sampling designs nested within a cohort illustrate sampling from a primary study base.

There are other types of primary study bases beyond research cohorts that are available. The most commonly used primary study bases in clinical research are defined by geographic areas for which there is a disease registry that captures nearly all of cases or defined by institutional membership, especially institutions such as health care entities that capture the relevant medical information and diagnoses.

Distinction Between Primary and Secondary Study Base

There is an important distinction between a primary and a secondary study base that occurs in attempting to define the population. If the study base can be clearly and explicitly defined as the members of a cohort, or the residents of a geographic area, or the members of a health care delivery system, we call that population a primary study base.

The advantage of being able to identify a primary study base is that there is no uncertainty about the population from which the controls should be selected. They should be selected from the same primary study base that gave rise to the cases.

Problems with Primary Study Base

Main problem with a primary base is often ascertainment of all cases Since there is no ambiguity about the population that gave rise to the cases when a primary study base can be identified, the problem for case-control studies is likely to be ascertaining all the cases for diseases without comprehensive registries. There may also be logistical problems with enrolling cases with poor survival because of the lag time between diagnosis and appearance in the registry. For example, a case-control study of glioma patients is likely to have difficulty contacting the cases quickly enough.