Sample Size Calculator with Incidence Rate
Calculate total contacts needed based on Incidence Rate (IR) and Response Rate.
Number of completed surveys required.
Percent of population that qualifies.
Percent of invites expected to participate.
Total Invitations Needed
0
Target Completes:
0
Estimated Screen-outs:
0
Project Feasibility:
–
Understanding Incidence Rate in Sample Size Calculations
In market research and survey sampling, the Incidence Rate (IR) is a critical metric that determines the feasibility and cost of a study. It represents the percentage of the general population (or your targeted list) that meets the specific criteria to qualify for your survey.
Why Incidence Rate Matters
If you need a sample of 1,000 people who own a specific rare car, and only 1% of the population owns that car, your Incidence Rate is 1%. This significantly impacts the Gross Sample—the total number of people you need to contact to get your desired number of completed interviews (Completes).
Lower incidence rates require sending out significantly more invitations, which increases the time, effort, and cost of data collection.
The Calculation Formula
To determine the total number of contacts (invitations) required, use the following logic:
Total Contacts = Target Completes / (Incidence Rate % × Response Rate %)
Example:
- Target Completes: 400
- Incidence Rate: 20% (0.20)
- Response Rate: 10% (0.10)
- Calculation: 400 / (0.20 × 0.10) = 400 / 0.02 = 20,000 Contacts Needed
Definitions
- Screen-outs: Participants who respond to the invitation but do not meet the criteria (based on Incidence Rate). These respondents are usually terminated early in the survey.
- Response Rate: The percentage of people invited who actually start the survey. This is distinct from incidence rate, which is about qualification.
- Completes: The final count of valid, fully answered surveys from qualified respondents.
Managing Low Incidence Rates
When dealing with an Incidence Rate below 5%, standard random sampling becomes very expensive. In these cases, researchers often utilize targeted panels or customer lists where the incidence rate is artificially higher because the population is pre-screened.