Incidence & Prevalence
Site: | EUPATI Open Classroom |
Course: | Statistics |
Book: | Incidence & Prevalence |
Printed by: | Guest user |
Date: | Sunday, 9 February 2025, 8:56 AM |
1. Prevalence & Incidence: Definitions
Definition of Prevalence (Harvard School of Public Health)
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Definition of Incidence (Harvard School of Public Health)
2. Understanding Epidemiologic Concepts
- How many are affected by such events?
- Is the risk increasing or decreasing?
- What is the relevance of the problem?
- How could it be prevented?
2.1. Definitions
- 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:
- at a specified point in time (e.g. on a certain date) – ‘point prevalence’, or
- 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.
- Incidence: The number of new cases of a health event during a given period in a specified population.
- Incidence rate: The frequency with which new health events occur, related to a particular time frame (e.g. the number of new cases per year). Incidence rate is worked out by dividing the number of new cases over a specified period either by:
- the average population (usually ‘mid period’ - the population half-way through the period being looked at), or
- ‘person-time’ - a measure of the number of persons at risk and the time they were at risk.
2.2. How Many People Are Affected?
|
Country A |
Country B |
Persons affected by a certain health event |
10.000 |
20.000 |
Total population of the country |
20.000 |
200.000 |

2.3. Example - Hair Colour
3. Prevalence


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:

- 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.
3.1. Calculating Prevalence
- 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.
4. Incidence


Often
you can read about the incidence of a heath event as a certain number of cases
per 100 or 1,000 person-years.
4.1. Calculating Incidence
|
Year 1 |
Year 2 |
Year 3 |
Year 4 |
Year 5 |
Patient 1 |
Healthy |
Healthy |
Healthy |
No data |
No data |
Patient 2 |
Healthy |
‘A’ |
Healthy |
Healthy |
Healthy |
Patient 3 |
Healthy |
Healthy |
Healthy |
Healthy |
Healthy |
Patient 4 |
Healthy |
Healthy |
Healthy |
‘A’ |
Healthy |
Patient 5 |
Healthy |
Died |
|
|
|


The result of 117.6 would be read as ‘117.6 new cases per 1,000 person-years’.