Incidence rate is the rate of qualified responses. With Google Surveys, it is the number of respondents who chose a target answer in the screening question.
A survey can have up to 10 questions including 1-4 screening questions. All surveys with screening questions require an audience test to ensure that the incidence rate of your survey is above our minimum rate (5%). Incidence rate is based on the rate of the last screening question in the survey (when there is more than one). If your survey is too targeted (e.g., with 4 screening questions), it may result in a lower incidence rate. We require surveys to meet a minimum incidence rate of 5% to provide valid data and enough responses.
The incidence rate for your survey based on this initial test dictates the custom price per completed response. When you set up a survey with a screening question, we test the survey for you and email you the price once we determine that we'll have enough users to complete your survey. You don't have to buy the survey until the audience test is complete and we've determined a price per completed response.