The incidence rate, sometimes called the Qualifying Rate, is the percentage of the population that will fully qualify and complete your survey after targeting is taken into account. Demographic parameters can be used to target certain respondents in the population and therefore increase your incidence rate.
Example: Your survey is looking for Females, Ages 18-54, who purchased carbonated beverages in the last 7 days. You are using parameters to target the appropriate age and gender. The percentage of those respondents, after targeting, who have purchased carbonated beverages in the last 7 days is your incidence rate. So, if 100 Female respondents ages 18-54 click on your survey and 25 of them have purchased carbonated beverages in the last 7 days, your incidence rate will be 25%.