Is there a way to easily implement a unique completion code for each participant? From the documentation on completion codes, it looks like you can only have 1 general code (which participants could then pass on to each other without having completed the study).
Dear @Katlijn_Haesebrouck, welcome to the Forum!
Your understanding of the completion code is correct. It identifies the study and cannot be provided differently for each participant within the same study. You can read more about it here:
As for your fear of cheating, this requires a lot of coordination among groups of participants that I would safely think is too costly if compared to the expected benefits of such an operation. Indeed, you must have participants that know each other (and are willing to cheat) to be invited to your study, which might not be the case because 1. your pre-screeners criteria might select only one of them, or 2. even if belonging to the same set of eligible participants, some might be left out because the study gets full before they get access to it (indeed, invitations are not sent out all at the same time but sequentially according to some algorithm that privileges less experienced participants).
If you fear instead some form of coordination among people that don’t know each other, you can always check the space on Reddit dedicated usually to Prolific participants and see if they talk about your study and share info such as the completion code there:
Anyway, even in the remote case when people manage to cheat in such a way, having a correct completion code does not impede rejection if the crucial parts of your study are skipped in their responses. See the permissible reasons for rejection here:
Hope this helps