How sure must you be when calling balls out? I mean, it seems a little pretentious to say you're 100% sure of anything. Afterall, you're going on your memory of what you thought you saw. It seems there is a slight chance that you're seeing what you want to see. So if we're not talking about 100% certainty, then what is acceptable? When we ask the opponent whether he's "sure" or not, what are we asking him to do but to reevaluate the basis of his decision, did he actually see court when the ball touched the ground, or how the ball came up off the court far from the line, etc.