Sue - do these practitioners operate more than 1 room simultaneously? (I.e. overlapping appointments?)
If not, then "intervals" are something that is set up for the app't book as a whole - so it sounds like you want your app't book to be set up in 15-minute intervals - whereas the "rules" that apply to practitioners are set up for each practitioner.
The first is:
System Info/Appointment Book/General tab/Calendar intervals.
The second is:
System Info/Appointment Book .. click the Rules button