ANTC67: Midterm Notes
o The severity of illness (if many people who develop a disease die within a short time, its prevalence is decreased)
o The duration of illness (if a disease lasts a short time its prevalence is lower than if it lasts a long time)
o The number of new cases (if many people develop a disease, its prevalence is higher than if a few people do so).
¾ Prevalence studies do not usually provide strong evidence of causality.
¾ Measures of prevalence are, however, helpful in assessing the need for preventive action, healthcare and planning of health services.
¾ Incidence refers to the rate at which new events occur in a population.
¾ In the calculation of incidence, the numerator if the number of new events that occur in a defined time period, and the denominator is
the population at risk of experiencing the even during this period.
population contributes one person-year to the denominator for each year (or day, week, month) of observation before disease develops,
or the person is lost to follow-up.
¾ Incidence (I) is calculated as follows:
I= # of new events in a specified period (x10n)
# of persons exposed to risk during this period
¾ The numerator strictly refers only to first events of disease.
¾ The units of incidence rate must always include a unit of time (cases per 10n and per day, week, month, year, etc).
¾ For each individual in the population, the time of observation is the period that the person remains disease-free.
¾ Since it may not be possible to measure disease-free periods precisely, the denominator is often calculated approximately by multiplying
the average size of the study population by the length of the study period.
¾ Cumulative incidence is a simpler measure of the occurrence of a disease of health status. Unlike incidence, it measures the denominator
only at the beginning of a study.
¾ The cumulative incidence can be calculated as follows:
Cumulative Incidence= # of people who get a disease during a specified period
# of people free of the disease in the population at risk at the beginning of the period
¾ Cumulative incidence is often presented as cases per 1000 population.
¾ In a statistical sense, the cumulative incidence is the probability that individuals in the population get the disease during the specified
¾ The period can be of any length but is usually several years, or even the whole lifetime.
¾ The simplicity of cumulative incidence rates makes them useful when communicating health information to the general public.
¾ Case fatality is a measure of disease severity and is defined as the proportion of cases with a specified disease or condition who die within
a specified time.
¾ It is usually expressed as a percentage.
Case fatality = # of deaths from diagnosed cases in a given period .
# of diagnosed cases of the disease in the same period
Interrelationships of the Different Measures
¾ Prevalence is dependent on both incidence and disease duration. It can be calculated approximately as:
P= incidence x average duration of disease
¾ Since incidence usually changes with age, age-specific incidence rates need to be calculated.
¾ The cumulative incidence rate is a useful approximation of incidence when the rate is low or when the study period is short.
Using Available Information to Measure Health and Disease