Incidence vs Prevalence: Definitions, Formulas and a Real Example

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Quick answerPrevalence is how many people in a population have a condition at a given time. Incidence is how many new cases appear during a period, among people who did not have it before. Prevalence is a snapshot; incidence measures the speed at which new cases arise.

These terms come from medicine and public health, but the idea applies to anything that spreads or is adopted. Below, we explain both using a real example from ScholarsColab’s data: how researchers started using machine learning.

Incidence vs prevalence at a glance

Prevalence Incidence
Question How many have it now? How many got it during this period?
Counts All existing cases, old and new Only new cases
Divided by The whole population The people who could still get it (the population at risk)
Typical study Cross-sectional survey Cohort study following people over time
Example 8% of adults in a district have diabetes 1 in 100 adults without diabetes develops it each year
A simple picture: think of a bathtub. Incidence is the water flowing in from the tap; prevalence is the water already in the tub. Recovery or death is the water draining out.

Example: machine learning among ScholarsColab mentors

To explain incidence and prevalence, we treat “has used machine learning in a published paper” like a condition, and our mentors as the population. Our data: 41,792 papers, mostly published between 1998 and 2026, by 967 ScholarsColab mentors (data updated 15 October 2026).

Step 1
Define the population. For each year, the population is the mentors who were already publishing by then.

Step 2
Measure prevalence. Count the mentors who had used machine learning in at least one paper up to that year, and divide by all publishing mentors.

Step 3
Measure incidence. Take only the mentors who had not used machine learning before that year (the population at risk), and count how many used it for the first time during the year.

Step 4
Compare the two.

YearMentors who had ever used machine learning (prevalence)Mentors who started using it that year (incidence)
201611.2% of 614 publishing mentors10 new, 1.9% of 515 who had not used it yet
201712.1% of 655 publishing mentors8 new, 1.5% of 545 who had not used it yet
201814.2% of 702 publishing mentors18 new, 3.1% of 576 who had not used it yet
201918.5% of 725 publishing mentors31 new, 5.1% of 602 who had not used it yet
202022.3% of 759 publishing mentors32 new, 5.4% of 591 who had not used it yet
202127.4% of 799 publishing mentors42 new, 7.1% of 590 who had not used it yet
202233.3% of 850 publishing mentors57 new, 9.8% of 580 who had not used it yet
202339.8% of 897 publishing mentors60 new, 10.6% of 567 who had not used it yet
202446.8% of 932 publishing mentors70 new, 13% of 540 who had not used it yet
202551.6% of 955 publishing mentors50 new, 10.1% of 496 who had not used it yet
In 2016, 11.2% of publishing mentors had ever used machine learning (prevalence); by 2025 it was 51.6%. In 2025 alone, 50 mentors used it for the first time: 10.1% of those who had not used it before (incidence).
Incidencenew users each year
→
Prevalenceeveryone who has used it

Step 5
Notice how they relate. As long as new users keep appearing (incidence above zero) and nobody “stops” having used it, prevalence can only rise. In disease, prevalence also falls as people recover or die, so a disease with short illness can have high incidence but low prevalence.

How to calculate them

  1. Prevalence = number of existing cases ÷ total population, at one point in time (point prevalence) or over a period (period prevalence).
  2. Cumulative incidence = number of new cases during a period ÷ population at risk at the start of the period.
  3. Incidence rate = new cases ÷ total person-time at risk (for example, cases per 1,000 person-years), used when people are followed for different lengths of time.
  4. Always state the time: “prevalence in 2025”, “annual incidence”.

Examples across fields

Field Prevalence Incidence
Public health Share of children who are anaemic New anaemia cases per year among children who were not anaemic
Engineering Share of bridges with cracks at inspection New cracks appearing per year among undamaged bridges
Management Share of firms using cloud software Firms adopting it for the first time each year
Agriculture Share of farms affected by a pest Farms newly affected each season

Common mistakes reviewers point out

  • Using the words interchangeably.
  • Dividing new cases by the whole population instead of the population at risk.
  • Not stating the time period.
  • Claiming incidence from a one-time survey, which can only measure prevalence.

Frequently asked questions

What is the difference between incidence and prevalence?

Prevalence is the proportion of a population that has a condition at a given time. Incidence is the number of new cases that develop during a period among people who did not have the condition before.

How is prevalence calculated?

Prevalence is the number of existing cases divided by the total population at a specific time or over a specific period.

How is incidence calculated?

Cumulative incidence is the number of new cases during a period divided by the population at risk at the start. An incidence rate divides new cases by the total time people were at risk.

Can prevalence be high when incidence is low?

Yes. Long-lasting conditions build up over time, so a slow rate of new cases can still produce high prevalence, as with many chronic diseases.

Which study design measures incidence?

Cohort studies, which follow people without the condition over time, are the usual way to measure incidence. Cross-sectional surveys measure prevalence.

Use this data in your own work

Use this data in your own research. Download the aggregated figures behind these charts (counts, medians and shares by type of paper, period, method and discipline) as a CSV file. Please cite it as:ScholarsColab (2026). ScholarsColab Research Methods Index [Data set]. Vidhilekha Soft Solutions Pvt Ltd. Retrieved 15 October 2026, from https://scholarscolab.in/research-methodology/. Licensed under CC BY 4.0.Download the data (CSV) →

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About the data on this page. The ScholarsColab Research Methods Index is built from the Google Scholar publication lists of PhD researchers who mentor on ScholarsColab, and is updated daily as mentors and publications are added. Methods, designs and paper types are identified from titles, so papers whose titles do not mention them are not counted in those groups. The mentors are not a random sample of all researchers (engineering and computer science are strongly represented), so the figures describe this group of researchers and may not apply to research as a whole.