Incidence and Prevalence
Here we look at two measures of disease in a population: incidence and prevalence.
- Incidence = # of new cases/# of people at risk in a given time frame
- Prevalence = # of people with the disease/# of people in the population
Incidence is really a measure of rate – how fast disease is accumulating in a population. Prevalence is a proportion and represents the disease burden, that is how many people have the disease.
Comparing Incidence, Comparing Risk
We can use incidence to compare the risk of getting a disease in two different populations. In this video, we compare the incidence of dying from gunshot wounds in various US cities in 2012. For your information, incidence that measures death is also called a mortality rate.
So where would you rather live? Chicago or Detroit?
Incidence Rates
Measuring incidence can be tricky. Patients are always leaving a study for different reasons. Some move away (“lost to follow-up”). Others die of other diseases (“competing risks”). This could make studying incidence difficult, unless you use incidence rates – or person-time incidence.
The Relationship Between Prevalence and Incidence
Prevalence and Incidence are related.
Prevalence = Incidence * Duration of Disease
We also look at the Epidemiologist’s bathtub to explain how the two are related.
Attack Rates and Case-Fatality rates
Another way to measure disease is to look at attack rates and case-fatality rates. These are both special cases of incidence. Attack rates are typically used in infectious disease epidemics. Here we look at getting diarrhea while on a cruise. Case-Fatality rates demonstrate the risk of dying from a disease. In the video we show how the risk of dying once being diagnosed with HIV has decreased over the years… and differs across states.
Seems you don’t want to live in Mississippi or Detroit. (My apologies to any citizens of those fine locations).
Test your comprehension
With this problem set on incidence and prevalence.
patwari 