Hi Mogli,
Ibex cannot know in advance how many participants you will end up running, so it cannot automatically assign groups based on your total number of participants.
Each participant is associated with one value from the Group column and only sees trials generated from rows with that value in the Group column. How that value is picked depends on how you set your experiment, as described in the original Ibex documentation. By default, PennController uses Ibex’s internal counter: in your case, your first participants would be assigned group A (the first value of the Group column in your spreadsheet) and after the first participant has completed your experiment, the next participant who takes the experiment would be assigned group B. After the second participant completes your experiment, the next participant taking the experiment would be assigned group C, etc.
If you like the internal counter method but would rather have the counter incremented as soon as a participant starts the experiment, you can use PennController.SetCounter.
You can replace experiment.html with server.py?withsquare=N in your experiment’s URL to force running the experiment in a specific group, where N is a number, from 0 to 15 in your case (see Ibex’s documentation).
Let me know if you have any questions
Jeremy