What is incidence rate? Examples of high and low incidence
Incidence rate is the percentage of people in a sample who qualify for a survey or study based on its screening criteria.
Incidence rate, in one sentence
Incidence rate is the percentage of people in a sample who qualify for a survey or study based on its screening criteria. It's calculated by dividing the number of qualified respondents by the total number of people screened, qualified and non-qualified combined. A high incidence rate means most people who are screened qualify, which makes a study faster and cheaper to run. A low incidence rate means only a small share of people qualify, which makes the same study slower and more expensive, since more people have to be screened out to find each usable respondent.
This guide covers the formula, what counts as high versus low in practice, and why the number matters well beyond a line item in a research proposal.
The incidence rate formula
Incidence rate = number of qualified respondents / (number of qualified respondents + number of non-qualified respondents)
For example, if a study screens 200 people and 50 of them meet the eligibility criteria, the incidence rate is 50 / 200, or 25%. The other 150 people were screened out because they didn't meet the study's requirements, whatever those happen to be, age, product usage, purchase recency, job title, and so on.
A general population survey, one with no screening criteria beyond being part of the general public, has an incidence rate close to 100%, since almost everyone who's invited qualifies. Add even one screening condition, and the rate drops from there.
Examples of high and low incidence
There's no single fixed cutoff for what counts as high or low incidence, but in practice, most researchers work with rough tiers like these:
High incidence, roughly 50% and above:
- General population surveys with no screening criteria
- Customer satisfaction surveys sent only to existing customers
- Broad demographic targets, such as all adults in a country
Moderate incidence, roughly 20% to 50%:
- Category users, such as people who bought a snack product in the last month
- Common life-stage targets, such as parents of school-age children
- Single-condition demographic splits, such as women aged 25 to 34
Low incidence, roughly 5% to 20%:
- Owners of a specific, less common product category, such as electric vehicle owners in a given city
- Multi-condition targets, such as women aged 25 to 34 who have purchased a specific product type in the last three months
- Users of a specific competitor brand
Very low incidence, under 5%:
- Niche B2B decision-makers, such as CFOs at companies above a certain revenue threshold
- Patients with a specific, uncommon medical condition
- People who've used a highly specific, newly launched product or feature
These tiers aren't fixed rules. The same criteria can land in a different tier depending on the country, category, or time period being studied. What matters is understanding roughly where a target audience falls before a study is designed, since that number drives almost every other decision that follows.
Why incidence rate matters
Incidence rate affects three things directly: cost, timeline, and sample size planning.
Cost. A low incidence rate means more people have to be screened out for every person who qualifies, and panel providers charge for that screening effort, not just for completed responses. A study targeting a 5% incidence audience will cost meaningfully more per completed response than a study targeting a 50% incidence audience, even if both studies collect the same final sample size.
Timeline. Reaching a fixed number of qualified respondents from a low-incidence population takes longer, since a much larger pool of people has to be invited and screened to find them. A general population study can often field in a day or two. A study targeting a rare, highly specific audience can take considerably longer, and in some cases isn't feasible to complete on a tight deadline at all.
Sample size planning. Incidence rate determines how large the initial invited pool needs to be to hit a target sample size. If a study needs 500 completed responses from an audience with a 10% incidence rate, the invited pool needs to be large enough to screen roughly 5,000 people, not 500, since 90% of them will be screened out along the way.
How to work with a low incidence rate
A few approaches are commonly used when a target audience has a low incidence rate:
- Broaden the screening criteria where possible, without compromising who the study actually needs to reach. Sometimes a screening condition is stricter than the research question actually requires.
- Use targeted recruitment rather than general population outreach, such as panels pre-profiled on the relevant criteria, so fewer people need to be screened out along the way.
- Adjust the timeline and budget expectations upfront, rather than discovering partway through fielding that a low-incidence study is taking longer or costing more than a standard one.
- Consider whether a smaller, statistically valid sample size is acceptable for a very low incidence audience, rather than defaulting to the same sample size used for higher-incidence studies.
Common mistakes with incidence rate
Assuming incidence rate stays constant across markets. An audience with a 30% incidence rate in one country can have a very different rate in another, depending on category penetration, demographics, or regulation.
Not accounting for incidence rate in project timelines. A study planned with a general-population timeline, then given a low-incidence screening criteria partway through scoping, is one of the most common causes of a research project running over schedule.
Treating incidence rate as fixed rather than estimated. Predicted incidence rate, based on available demographic or category data, is an estimate. The actual incidence rate, confirmed once fielding starts, can come in higher or lower, and budgets built with no margin for that difference tend to run into trouble.
Summary
Incidence rate is the percentage of people who qualify for a study out of everyone who's screened for it, and it's one of the most practical numbers in research planning, because it drives cost, timeline, and sample size all at once. A high incidence rate audience is fast and cheap to study. A low incidence rate audience, especially anything under 5%, needs a longer timeline, a larger invited pool, and a bigger budget to reach the same number of completed responses.
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