Study Base

The study base is the population who experienced the disease outcomes you will observe in your study.

• In a Cohort Study, the study base is an explicitly defined cohort.
• In a Cross-Sectional Study, the study base is a hypothetical cohort sampled at one point in time.
• In a Case Control Study, the study base is the cohort, either explicit or hypothetical, that gave rise to the cases.

In a cohort study, the study base is an explicitly defined group of individuals based on some set of characteristics at a given time, called time zero. This group of individuals is then followed forward in time. We will look at cross-sectional and case-control studies in the setting of a cohort because a cohort study explicitly defines its study base as the individuals who are recruited into the cohort. The members of the cohort are the population whose disease experience will be observed during the study follow-up period. Looking at cross-sectional and case-control studies in the setting of a cohort makes clear how they sample (or do not sample!) the cohort study base.

By a hypothetical cohort we mean that a cohort study was not performed, but that some possible or hypothetical cohort of individuals among whom the disease diagnoses were made can be defined. That group of individuals could in theory have been enrolled in a cohort study. An example would be the members of the Kaiser Permanente HMO at some point in time. If all the members of Northern California Kaiser Permanente, say, had been enrolled in a cohort study of a particular disease five years ago, measurement of the disease outcome and possible predictors of disease during the past five years would provide cohort study data. Even outside a well defined population like an HMO, there is always a hypothetical cohort among whom the cases would have been diagnosed if they had been enrolled in a cohort study. For example, if some group of persons with the disease diagnosis during a five-year period had been enrolled in a cohort at the beginning of the five-year period along with a sample of their neighbors who were not diagnosed with the disease, they would constitute a cohort. In practice, it may be difficult to identify this kind of less well defined hypothetical cohort.