Knowing how to calculate survey incidence is essential for planning the time, sampling, and costs of any study that requires qualified recruitment.

In short, incidence indicates the proportion of people who meet the project criteria among all people evaluated. Thus, with this measure in hand, you decide how many interviews you will need to start to reach the desired final sample.

What is incidence and why it matters

First, incidence is the eligibility rate: it represents how many people, among all those you approached, actually meet the study's inclusion criteria. In other words, it is the measure that reveals how common — or how rare — it is to find the audience you need within a larger universe of potential participants.

Then, as this rate becomes clear, it directly helps in estimating operational effort. This happens because the lower the incidence, the greater the number of contacts needed to find qualified people, and consequently, the higher the cost and time dedicated to the field. On the other hand, when the incidence is higher, recruitment tends to be more agile, reducing both the volume of screening and the investment needed to reach the final sample.

Furthermore, as incidence is revealed during screening, it also guides important strategic decisions about recruitment. For example, if the rate is very low, the team may choose to change channels, broaden criteria within methodological limits, or reinforce incentives. If, on the contrary, the incidence is above expectations, it is possible to optimize resources and speed up steps that previously seemed longer.

Finally, understanding incidence allows for adjusting expectations, predicting bottlenecks, and making decisions with more confidence. In other words, it functions as a compass for planning, monitoring, and adjusting the entire data collection process.

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Basic formula

The formula for calculating incidence is simple and direct, but its impact on research planning is enormous. It serves as a starting point for any estimate of effort, cost, and field time. See:

Incidence (%) = (Number of eligible people ÷ Total number of people interviewed/evaluated) × 100

To make it even easier, think of it this way: first, you identify how many people actually meet the study's criteria. Then, you compare this number with the total number of people evaluated during the screening process. Next, simply divide one by the other and, finally, multiply by 100 to transform this proportion into a clear and immediately understandable percentage.

In other words, you divide the number of eligible people by the total verified and multiply by 100 to obtain the final incidence. In this way, the calculation objectively reveals the representativeness of the target audience within the evaluated universe. Furthermore, this percentage serves as a solid basis for predicting how many additional contacts will be needed to reach the desired sample.

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Practical step-by-step to calculate incidence

1. Collect raw data: Before any calculation, it is essential to gather the basic information that will serve as the basis for incidence. Therefore, record how many people you evaluated (screened) and how many were actually considered eligible to participate in the study. This initial step ensures that the entire process is built on consistent and verifiable data.
2. Calculate the ratio: Next, with these numbers in hand, proceed to calculate the ratio between eligible and screened. Here, simply divide the total number of eligible people by the total number of people evaluated. This action transforms the screening into an initial proportion that already begins to indicate the level of difficulty of recruitment.
3. Convert to percentage: After that, to make the information more intuitive and facilitate comparison between different studies, multiply the result by 100. Thus, you convert the ratio into a clear percentage — and it is this percentage that represents the final incidence.
4. Use incidence for forecasting: Finally, use this value to project the effort needed for the rest of the research. From the incidence, you estimate how many contacts you will need to make to obtain the desired sample. In this way, the calculated number ceases to be just a statistic and begins to guide the operational, financial, and methodological planning of the study.

To visualize better, consider this example: suppose you screened 2.000 people and, among them, 200 were eligible. In this case, the calculation is as follows:

Incidence =(2000200​)×100=10%

Therefore, only 10% of the screened people met the criteria.

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Practical applications

1 – Budget: Incidence also directly influences the cost of the study. When you multiply the number of necessary contacts by the cost per contact, you get a more realistic forecast of the investment in the field. In this way, incidence helps avoid financial surprises and allows you to adjust the budget even before starting recruitment.

2 – Schedule: Furthermore, knowing the incidence allows you to anticipate the pace of data collection. By estimating how many people can be screened per day, you can more accurately predict how many days will be needed to complete the desired sample. Thus, the schedule ceases to be a gamble and begins to reflect the real capacity of the operation.

3 – Methodology adjustment: Finally, incidence also guides strategic decisions. If it is too low, it is a sign that the process can become slow and expensive. In these cases, it is worth considering alternatives: expanding recruitment channels, making inclusion criteria more flexible (whenever methodologically possible), or resorting to specialized panels that already have a qualified audience. These adjustments can speed up the field and avoid operational wear and tear.

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Common errors and precautions when calculating incidence

• Do not confuse incidence with response rate: Often these two indicators seem similar, but they measure different things. The response rate shows how many people accepted to participate among those you invited. The incidence reveals how many, among all those screened, actually meet the eligibility criteria. In other words, a survey can have a great response rate and still have a low incidence — and vice versa.
• Avoid estimating incidence with a very short screening: When the initial screening volume is small, any specific variation can distort the estimate. Therefore, it is important to gather a minimum number of cases before calculating incidence. This way, you avoid hasty conclusions and reduce the risk of overestimating or underestimating the actual effort needed for the field.
• Recalculate incidence periodically: Throughout data collection, the sample's behavior can change — especially in segmented studies or with multiple profiles. Therefore, reviewing incidence in regular cycles allows for adjusting projections, adjusting schedules, and, if necessary, reorganizing recruitment strategies. In this way, planning remains aligned with the reality of the field and avoids unexpected delays.

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If your research requires reach, quality, and qualified recruitment, PainelTAP offers complete panels and structured processes so that each stage is conducted safely and quickly.
Talk to PainelTAP and discover how your next studies can be faster, more efficient, and more reliable.