I ran a pilot stage for my experiment and received numerous messages from people telling me that the "Session is full." I'm using oTree HR to run the study with Prolific. I only set the room for 10 participants, so are the people who get this message trying to run the study after all 10 participants have already completed it? How can I avoid this?

Thanks.

If you want to run a study online with x participants, you should set the room size to a number that is (substantially) larger than x.

The reason is that many e-mail clients and apps like WhatsApp will try to create a “screenshot” of a webpage when they encounter a URL. For this purpose, the e-mail client/app will access the URL. In an oTree room, if you distribute only the room-wide URL, each such access will take up one slot.

See https://www.otreehub.com/forum/1258/.

Another solution would be to use person-specific URLs. That is, add ?participant_label=INDIVIDUAL_LABEL_GOES_HERE to the room-wide URL. See https://otree.readthedocs.io/en/latest/rooms.html#using-rooms.