Prevalence and incidence are two ways to measure the number of people who have a certain disease in a population.
Prevalence includes all individuals with a disease, or all cases, regardless of when they developed it, divided by the total population.
Whereas incidence includes the new cases of a disease - meaning anyone who develops a disease, over some period of time, divided by the population of individuals that can get that disease.
And both of these have to take into account the population at risk.
Sometimes incidence and prevalence are reported as a proportion of a population like 5 per 1000, or 5 per 100, 000 if it’s a rare disease.
Let’s use a quick analogy to show the relationship between incidence and prevalence.
Imagine a bucket of green marbles - with each marble representing a sick person.
New green marbles keep falling into the bucket - these are the new cases.
We can count up how many green marbles enter the bucket in a year, and that would be the incidence.
Now some marbles also fall out of the bucket - perhaps some cases recover from the disease while other cases die.
Either way, they don’t have the disease anymore and are no longer cases in our bucket.
We can count up the number of green marbles in the bucket and that would be our prevalence.
So the prevalence really depends on the incidence of new cases, the recovery rate of the cases, and the death rate of the cases.
So for prevalence we have all cases of a disease over the population at risk.
If it’s done at some specific moment, like the day that a survey is being done, then it’s called point prevalence.
And occasionally, if it’s done over a period of time, like over a month or year, then it’s called period prevalence.
Now when we have new cases of a disease in a certain period of time, it’s called incidence rate.
This can’t be done at a specific moment, since we need some time window to capture the new cases, and sometimes it’s hard to follow up with everyone for the entire study period.
When this happens, we can use something called person time to find the incidence rate.
That’s when you sum up the time observed for every person during the study period.
Let’s look at an example of incidence rate using person-time.
Let’s say we want to figure out the incidence rate of pneumonia in a small retirement home that has 10 people, and that we do a 3 year study.