European Patients’ Academy
on Therapeutic Innovation
http://www.eupati.eu
Fact Sheet: Prevalence
Definitions
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Health event: A particular disease, injury, other health condition or attribute.
Population: The total number of persons living in a particular place (e.g. a town or
country), or the total number of persons in a particular group (e.g. with the same job
or educational background).
Prevalence: The total number of cases of a health event in a specified population.
Prevalence rate: The proportion of a population that has a health event:
o at a specified point in time (e.g. on a certain date) – ‘point prevalence’, or
o during a specified period (e.g. over 12 months) – ‘period prevalence’.
Proportion: A ratio between health events occurring and population, often shown as
a fraction or percentage. For example, the number of people who have a disease
compared with the total number of people studied.
Usually the first question about a certain health event will be: How many people are
affected? or What is the prevalence?
Depending on the population we are looking at, the answer will differ. Let’s say that there are
10.000 cases in country A and 20.000 cases in country B. Could we state that the health
problem is far more serious in country B? Let’s have a look on the following table:
Country A
Country B
Persons affected by a
certain health event
10,000
20,000
Total population of the
country
20,000
200,000
In country A, a total of 10,000 out of a population of 20,000 are affected, which means that
50% are affected by the health event.
In country B, a total of 20,000 out of a population of 200,000 are affected, which means that
10% are affected by the health event.
The mathematical way to calculate this would be:
𝐍𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐩𝐞𝐫𝐬𝐨𝐧𝐬 𝐚𝐟𝐟𝐞𝐜𝐭𝐞𝐝 𝐛𝐲 𝐚 𝐡𝐞𝐚𝐥𝐭𝐡 𝐞𝐯𝐞𝐧𝐭
× 𝟏𝟎𝟎
𝐓𝐨𝐭𝐚𝐥 𝐩𝐨𝐩𝐮𝐥𝐚𝐭𝐢𝐨𝐧
= % 𝐨𝐟 𝐩𝐨𝐩𝐮𝐥𝐚𝐭𝐢𝐨𝐧 𝐚𝐟𝐟𝐞𝐜𝐭𝐞𝐝 𝐛𝐲 𝐡𝐞𝐚𝐥𝐭𝐡 𝐞𝐯𝐞𝐧𝐭
With this formula we get information about the percentage of a population that is affected by
a health event.
1
European Patients’ Academy
on Therapeutic Innovation
http://www.eupati.eu
While our first impression would be that 20,000 are twice as many as 10,000 (which is
definitely true!), we now have a very different picture, as we put the numbers in relation to the
population.
Understanding these basics will make the concept of prevalence easier to understand. The
prevalence compares the number of persons having a certain characteristic with the total
population.
Example – hair colour
Imagine that a while ago you were celebrating a party with friends. You look back at a group
photo and notice that some of your friends have their hair coloured red. There were 100
guests and 30 of them had red hair – the prevalence of red hair at the party was 30%.
If we were talking about a health event, we would ask: ‘What is the prevalence of that health
event?’, but the mathematical calculation would be exactly the same. We could express the
prevalence as percentages (here 30%) or alternatively as cases per 1,000 persons (here 300
per 1,000).
Co-morbidity
In a population of 10,000 patients having a certain disease, 50 persons are reported to be
affected by another disease in addition. This is called a ‘co-morbidity’. So what is the
prevalence of the co-morbidity in this population?
The mathematical way to calculate this would be:
𝐧𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐜𝐚𝐬𝐞𝐬
× 𝟏𝟎𝟎
𝐩𝐨𝐩𝐮𝐥𝐚𝐭𝐢𝐨𝐧
This formula will provide us with the information as a percentage. By dividing 50 by 10,000
and multiplying the result by 100 (to make it a percentage) we find out that 0.5% of the
population is affected. So the prevalence of the co-morbidity is 0.5% in our population.
50
× 100 = 0.5%
10.000
Rather than expressing prevalence as a percentage, we can describe it as the number of
people affected in a standard sized population, for example 1,000 people. So instead we
would calculate:
50
× 1.000 = 5
10.000
This means that, for each 1,000 patients, 5 of them have co-morbidity.
Keep the example of the party in mind. Prevalence is like describing a group photo:
 How many people can you see there? That number is your population.
 How many people share a certain feature (e.g. same hair colour)? This number is
used to calculate prevalence.
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European Patients’ Academy
on Therapeutic Innovation
http://www.eupati.eu
Calculating prevalence
In epidemiology we actually have three different ways to calculate the prevalence:
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Point prevalence: the number of cases of a health event at a certain time. For
example, in a survey you would be asked if you are currently smoking.
Period prevalence: the number of cases of a health event in reference to a time
period, often 12 months. For example, in a survey you would be asked if you have
smoked during the past 12 months.
Lifetime prevalence: the number of cases of the health event in reference to the
total lifetime. For example, in a survey, you would be asked if you have ever smoked.
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