Measures of disease frequency

Question 1

Incidence

Answer 1

Incidence = the number of newly diagnosed cases of a disease that occur within a specific time frame.   Cumulative incidence (aka incidence proportion, risk, and average risk)
Incidence rate (aka person-time incidence rate, incidence density)

Question 2

Cumulative incidence

Answer 2

Cumulative incidence = the proportion of a candidate population that becomes diseased over a specified period of time

Note that the numerator (new cases of disease) is a subset of the denominator (candidate population), and so the possible value of cumulative incidence ranges from 0 to 1 or, if expressed as a percentage, from 0 to 100%.

Cumulative incidence can be thought of as the average risk of getting a disease over a certain period of time. (A risk is the probability of getting a disease.) A commonly cited measure of cumulative incidence is the lifetime risk’.

Cumulative incidence is usually reserved for fixed populations, particularly when there are no or few losses to follow-up.

Question 3

Incidence rate

Answer 3

Incidence rate = Incidence rate is defined as the occurrence of new cases of disease that arise during person-time of observation.

Note that the numerator for incidence rate is identical to that of cumulative incidence. The difference between the two measures lies in the denominator. The incidence rates denominator integrates time (t), and so it is a true rate. Thus, its dimension is 1/t, and its possible values range from zero to infinity. An incidence rate of infinity is possible if all members of a population die instantaneously.

The concept of person-time can be difficult to understand. Person-time is accrued only among candidates for the disease. Thus, a person contributes time to the denominator of an incidence rate only up until he or she is diagnosed with the disease of interest. However, unlike for cumulative incidence, the incidence rate is not based upon the assumption that everyone in the candidate population has been followed for a specified time period. Person-time is accrued only while the candidate is being followed. Accrual of person-time stops when the person dies or is lost to follow-up (such as when a person moves away from a community).

The incidence rate can be calculated for either a fixed or dynamic population. However, because it directly takes into account population changes (such as migration, birth, and death), it is especially useful as a measure of the transition between health and disease in dynamic populations.

Question 4

Prevalence

Answer 4

Point prevalence

Period prevalence

Question 5

Prevalence

Answer 5

Prevalence is a proportion and is a measure of the burden of disease within a population.

Point prevalence = number of cases in a defined population / number of people in a defined population at the same time

Period prevalence = number of identified cases during a specified period of time / total number of people in that population
Prevalence = incidence x duration of condition
In chronic diseases the prevalence is much greater than the incidence
In acute diseases the prevalence and incidence are similar. For conditions such as the common cold the incidence may be greater than the prevalence
Incidence is a useful measure to study disease etiology
Prevalence is useful for health resource planning