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Actually, this is where a lot of people blow it.
A roll is supposed to be to school the dog, not necessarily to judge him yet.
Your first few rolls should be against dogs of different styles, to teach your dog how to handle them, and these rolls should be fairly short (< 15 min). You really shouldn't be passing judgment here, but instead assuming the role of educator.
Only after you first invest the time allowing your dog to mature, and educating your dog in the process by exposing him to dogs of various styles, should you then place him in a serious hard roll (or match) to evaluate him for real. Too many people want to game test their dogs damned near the same day they first start, which is a mistake.
Naturally, during the schooling process, if the dog is an ace (or a bum) you can see these levels of ability as you go along ... but his gameness shouldn't be seriously challenged at all ... until he is FULLY-mature, FULLY-started, and FULLY-schooled
Jack