The Participant Identifier links the email address to the survey responses. It can only be used when the first instrument on a project is enabled as a survey. To use it, do the following:

Go to "Participant List" and click on the "Enable" button that's next to "Participant Identifier" (shown in image below)
Click "Yes, ENABLE Participant Identifiers"
You can then add an identifier by clicking on a blank box under "Participant Identifier"
When you click on "Add participant", a box similar to the one in the image below will appear. Enter their email address, followed by a comma, then enter the participant identifier (can be name, ID number, or anything else that specifies the record). Examples #2 and #3 in image below demonstrate on how to enter it.
You can then compose the survey invitations. When a participant responds, you'll be able to click on the green checkmark circle (circled in red in image below) and open up the survey (can only open it this way when "Participant Identifier" is enabled.)

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