What is the difference between the Public Survey Link and the Private Survey Link?

There are two different ways to invite individuals to participate in a survey. Each method was designed for a specific purpose. Which method you choose depends upon the requirements of your project. If you are concerned about security of the responses, then I would highly recommend using the Private Survey Link option. Both methods are listed below:

Public Survey Link
The Public Survey Link may be the easiest way to promote your survey. The attributes of this survey type are:
1. Responses are anonymous (unless the survey contains identifying questions).
2. All participants access the same URL.
3. The same participant may take the survey multiple times.
4. There is no way monitor responses as you have no way to tell who did or did not fill out the survey(s).

Private Survey Link
The Private Survey Link offers the greatest control over the survey responses. The attributes of this survey type are:
1. Responses are anonymous (unless the survey contains identifying questions).
2. A unique URL is generated for each participant.
3. Each participant may respond to the survey only once.
4. You can send directly to your target audience.
5. You may view the status of each survey via the Survey Invitation log.
As for the management of the responses within the project, that is governed by the access that has been granted to each user within the project. Clearly, if someone has been granted access to export the data and then chooses to send that data on to someone else, then it is no longer secure. REDCap provides a secure and web-based tool that allows users to input data from anywhere in the world with secure web authentication, data logging, and secure sockets Layer (SSL) encryption. It is hosted here at CCHMC, so you don't need to worry about security issues regarding third party websites hosting your private data. However, it is important to keep in mind the security of any project relies heavily on those who have been granted data management privileges within the project as it is possible for data to be exported by those who have been granted the ability to do so.

Note:

If you want to utilize the Public URL to distribute an initial survey and invite participants, the survey MUST contain a text field with validation = email to collect the participants email address.

On the Project Setup page > Enable optional modules and customizations > Enable: Designate an email field to use for invitations to survey participants. Designate the email address you are collecting on the first survey.

When participants complete the first survey, their email addresses will pre-populate the Participant Lists and will allow you to send additional surveys for the same record.

Surveys will be automatically linked by record ID. Participant Identifier on Participant List will not be editable.

Additional Notes: You will still be able to use the Participant List to send emails to the first survey, if needed. Participant will be prompted to enter their email address on the survey itself. You can also create new records using the Data Entry feature to populate the first survey and manually enter email addresses.

More information regarding this setup can be found here.