While there are many measures of disease frequency, epidemiologists most often estimate the occurrence of disease in a population in terms of incidence or prevalence of the disease. The key difference between these two measures is the time of disease onset—are existing cases counted or only new cases?

**Incidence**: counts *new* cases of the disease (or outcome)

**Prevalence**: counts *new* *and* *existing* cases of the disease (or outcome)

**Incidence** **(new** **cases)**

Incidence quantifies the development of disease. Incidence can be estimated using data from a disease registry data or a cohort trial. There is an implicit assumption of a period of time, such as new cases within a month (or a year).

A summary incidence rate can estimate the risk (e.g., probability of disease in an individual) if the risk is constant across the summarized groups.

As defined, incidence is a count of new cases. However, it is often expressed as a proportion of those at risk. The denominator includes all persons at risk for the disease or condition, i.e., disease-free or condition-free individuals in the population at the start of the time period. Persons in the denominator, those at-risk, should be able to appear in the numerator. Obviously, the denominator would not include persons who already have the disease or condition. Incidence can also be expressed in terms of person-time at risk.

Rates are usually expressed per 100, 1,000, or 100,000 persons. In a strict application, “rate” should only be used when the denominator is an estimate of the total person-time at risk. (You will find the term “rate” used inconsistently in epidemiologic reports. It is better to seek the source of the numbers than to rely on the nomenclature.)

**Prevalence** **(new** **and** **old** **cases)**

Since prevalence counts both new and existing cases, the duration of the disease affects the prevalence. Diseases with a long duration will be more prevalent than those with a shorter duration. Chronic, non-fatal conditions are more prevalent than conditions with high mortality.

Similar to incidence, persons included in the denominator must have the potential for being in the numerator, i.e., at-risk for the disease or condition. Prevalence is often expressed after multiplication by 100 (%), 1000, or 100,000.

The term rate may be inappropriately applied to prevalence. Prevalence is a proportion, usually reflecting the proportion with a disease at a particular time. (Point prevalence)

The **prevalence** **pool** is the subset of the population with the condition of interest. The prevalence pool is not generally useful for hypothesis-driven epidemiologic research because these are not new cases, but can be useful in tracking the natural history of the disease, evaluating effects of treatments, or disease burden.

For most etiologic research, incidence is the more appropriate measure. Studying the incidence of a rare condition, however, poses a challenge. Given a small number of new cases, it can be preferable to estimate prevalence instead of incidence in these situations. For example, birth defect rates reported as the number of cases/live births is a prevalent measure. Similarly, an autopsy rate is a prevalent measure. 12