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Suppose that you are offering respondents an incentive to complete your survey; for example, a chance to win one of four $25 gift cards. Your survey promises the respondents that their responses will be anonymous; however, you will need to obtain some identifiable information about them in order to distribute the incentives, whether it's an email address (if sending a digital code for a gift card) or a physical address (if physically mailing a gift card).
To obtain respondent information and still keep survey responses anonymous, you will need to create two surveys: your “actual” survey and a second “incentives” survey that will collect only the information that’s necessary to deliver the incentive to the participants. This way, the survey responses and the contact information are stored in two separate “pools”, and will not be linked to each other.
Edit (June 2020): Researchers implementing this approach should be aware of the potential for abuse from individuals attempting to fill out the incentives survey more than once. We have added an additional step to these instructions (step 1.3), which will restrict access to the incentives survey to only those individuals who are coming from the "main" survey. (Stated differently, it will prevent individuals from directly navigating to the incentives survey by sharing the incentive survey URL.) This cannot completely prevent abuse, but implementing this step will make it so that only individuals referred by your "main" survey can access the incentives survey. We also recommend that researchers review Qualtrics's page on Survey Protection options.
Locate the option Add a referral website URL (previously called "HTTP Referer" or "HTTP Referer Verification" and change its setting to On. Erase the contents in the text box that appears, then copy and paste the URL of your main survey into the box. (If you have not yet published your main survey, your main survey will not have a URL assigned. You will need to get that URL in order to complete this step. If you are not yet finished setting up the main survey, you can temporarily publish it just to obtain its URL, then unpublish the survey to continue working on it.)
When you have completed setting up the branch logic, the Survey Flow window should look similar to this: