Incidence (epidemiology) facts for kids

Incidence is a way to measure how often a new health condition or event happens in a group of people over a certain time. It helps us understand the chance of someone getting a new illness.

Imagine a group of people. Incidence tells us how many of them get a new disease during a specific period, like a year. It's usually shown as a proportion or a rate.

What is Incidence Proportion?

Incidence proportion, also called cumulative incidence, is the chance that a specific event, like getting a certain disease, will happen to someone before a certain time.

To figure it out, you divide the number of new cases of a condition by the total number of people who were at risk of getting it at the start of the study.

For example, if you have 1,000 people and 28 of them get a new condition over two years, the incidence proportion is 28 out of 1,000 people. This is the same as 2.8%.

When we look at this chance over a person's whole life, it's called lifetime risk.

What is Incidence Rate?

The incidence rate measures how often a new disease or event occurs over a specific time. It's also known as incidence density rate.

This rate uses something called person-time in its calculation. Person-time is the total amount of time all the people in the study were observed. For example, if you watch 10 people for 1 year each, that's 10 person-years.

Let's use the same example: if 28 people out of 1,000 got a condition over two years, the incidence rate would be 14 cases per 1,000 person-years. This is because the 28 cases are spread over two years for 1,000 people.

Using person-time helps when people are observed for different lengths of time. It also helps if the number of people at risk changes over time.

Incidence vs. Prevalence

It's easy to mix up incidence with prevalence, but they are different.

• Incidence tells you about the risk of getting a new condition. It focuses on new cases.
• Prevalence tells you how widespread a condition is at a specific moment. It counts all existing cases.

Think of it this way:

• Incidence is like the number of new students joining a school each year.
• Prevalence is like the total number of students in the school right now.

Prevalence helps us understand how much of a burden a condition puts on society. Incidence is often more helpful for understanding what causes a condition. For example, if the incidence rate of a condition goes up, it might mean a new risk factor is causing more people to get sick.

A simple way to think about it is: Prevalence = Incidence × Duration This means if a condition starts happening more often (higher incidence), its prevalence will also likely increase.

Example: Understanding a Health Condition

Let's look at an example to make this clearer. Imagine you are studying a group of 225 people over 10 years to see how many develop a certain health condition.

• At the start of the study (year 0), you find 25 people already have the condition. These people are not counted for new cases because they already have it.
• After 5 years, you find 20 new people have developed the condition.
• After 10 years (the end of the study), you find 30 more new people have developed the condition.

If you wanted to find the prevalence at the end of the study, you would add up all the cases (25 existing + 20 new + 30 new = 75 total cases). Then you divide by the total group size (75 / 225 = 0.33 or 33%). This tells you that 33% of your group had the condition by the end of the study.

Now, let's find the incidence rate. This is more complex because we need to consider how long each person was healthy before getting the condition.

• The 20 people who got the condition by year 5 are assumed to have gotten it around 2.5 years into the study (halfway between year 0 and year 5). So, they contributed 20 people × 2.5 years = 50 person-years of healthy life.
• The 30 people who got the condition between year 5 and year 10 are assumed to have gotten it around 7.5 years into the study (halfway between year 5 and year 10). So, they contributed 30 people × 7.5 years = 225 person-years of healthy life.
• There were 150 people who never developed the condition over the whole 10 years. They contributed 150 people × 10 years = 1500 person-years of healthy life.

Add up all the healthy person-years: 50 + 225 + 1500 = 1775 person-years.

Now, take the total number of new cases (20 + 30 = 50 new cases) and divide by the total person-years (50 / 1775 = 0.028).

This means the incidence rate is about 28 new cases per 1,000 people per year. In simpler terms, if you followed 1,000 healthy people for one year, you would expect to see about 28 new cases of this condition. This gives a much better idea of the actual risk of developing the condition.

See also

• Attack rate
• Attributable risk
• Denominator data
• Rate ratio