Hi,

I need to conduct a longitudinal study with 3 data collection moments and I am not finding a suitable solution to link and distribute my 3rd survey at the correct time. I am hoping to receive some suggestions from the Community. 

Here is what I would like to do:

Survey 1→ Respondents fill in the consent form to participate in the study, including identifiable information (name, email address).

Survey 2→  Survey 2 directly follows Survey 1 (via URL or email task) and is linked to Survey 1 through an embedded “participant ID” (which was randomly generated in Survey 1). Survey 2 can be filled in right after Survey 1. In Survey 2, I ask participants to indicate when their therapy session will take place.

Survey 3 → Here is what I cannot solve: I want to send Survey 3 to participants exactly the day after their therapy session. This day will be different for everyone, so I cannot automatize this and schedule a fixed time for everyone based on some rule. I do not mind sending Survey 3 manually (checking, for each participant, the correct date, based on the responses of Survey 2), however, I need to be able to link the data of Survey 3 to the data of the first two surveys. I also need the data from Survey 3 to be collected (pseudo)anonymously (i.e., without asking for any identifiable information). How can I do this? Can I generate a personal link for each participant that would include the same embedded data (participant ID) that was included in the first two surveys and can I send this link myself whenever I want (not with email tasks)? 

I know that I could work with authenticators and anonymous links, by letting participants know what their participant ID is and asking them to use it to authenticate themselves in Survey 3. However, since a long time might pass between Survey 1/2 and 3, I am afraid they will forget/lose the participant ID. 

I hope my question is sufficiently clear. Any suggestions would be greatly appreciated! Many thanks in advance.