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I think it would depend on who is compiling the statistics and what their agenda is.

Seconded.

Recording "mix" bites is hard because some mixes are known to the owner, while others are a guess, or up in the air. I know Roxy's mix because they only had two intact dogs when I went to get her and they had had litters between them before. Sometimes there's multiple possible sires in situations, or guesses as to breed are made solely based on appearance.