In epidemiology, incidence reflects the number of new cases of a given medical condition in a population within a specified period of time.
For example, if a population contains 1,000 persons and 28 develop a condition from the time the disease first occurred until two years later, the cumulative incidence is 28 cases per 1,000 persons, i.e. 2.8%.
The incidence rate can be calculated by dividing the number of subjects developing a disease by the total time at risk from all patients:[1]
One of the important advantages of incidence rate is that it doesn't require all subjects to be present for the whole study because it's only interested in the time at risk.
Prevalence is the proportion of the total number of cases to the total population and is more a measure of the burden of the disease on society with no regard to time at risk or when subjects may have been exposed to a possible risk factor.
Prevalence can also be measured with respect to a specific subgroup of a population.
For example, consider a disease that takes a long time to cure and was widespread in 2002 but dissipated in 2003.
In contrast, a disease that has a short duration may have a low prevalence and a high incidence.
Say you are looking at a sample population of 225 people, and want to determine the incidence rate of developing HIV over a 10-year period: If you were to measure prevalence you would simply take the total number of cases (25 + 20 + 30 = 75) and divide by your sample population (225).
To measure incidence rate you must take into account how many years each person contributed to the study, and when they developed HIV because when a subject develops HIV he stops being at risk.
When it is not known exactly when a person develops the disease in question, epidemiologists frequently use the actuarial method, and assume it was developed at a half-way point between follow-ups.
[citation needed] In this calculation: That is a total of (1500 + 275) = 1775 person-years of life.
In other words, if you were to follow 1000 people for one year, you would see 28 new cases of HIV.