I have noticed this occuring on a number of surveys and it has happened on a survey we are currently running.
I run all our surveys with the following settings: Survey authentication: Password assigned by invitations Allow multiple submissions: No, prevent with: invitations Anonymity of respondents in invitations: No anonymity
When viewing the Manage responses page, I occasionally see a logged response with a blank email (i.e. the invitee email address). Opening the response shows all of the imported data (including pre-population information) but no email address.
1. How is this possible? 2. Is there any way I can track back to identify the invitee who made the survey response? 3. If the email address has not been registered by the system, is it possible for that (unknown) invitee to respond a second time to the survey?