ABSTRACT
This chapter reviews epidemiological measures of disease frequency and association, and how they relate to the underlying study design. Common measures of disease frequency include prevalence and incidence, where prevalence is a measure of the number of existing cases of a disease or condition and incidence is a measure of the number of new cases of a disease or condition. The chapter pays particular attention to distinguishing between cumulative incidence and incidence rates, along with an examination of follow-up time in cohort studies and how this impacts EHR research. Ratio measures of association are discussed next in the chapter, including the relative risk and the odds ratio, both of which are useful when conducting etiologic research on a health outcome. If one or more causal factors are identified, absolute, or different measures of association indicate the potential benefits of a healthcare intervention. The chapter discusses all of these measures and their calculations in the context of hypothetical cross-sectional, cohort, and case-control studies as defined in the previous chapter.
