If you can't prove, you can't know. If there is no measurable number of provable instances, you really can't claim it happens.
That's kind of my point Boo...
I imagine it's reasonable to suggest Fraud is happening to some degree, . The human condition suggests there will always be someone attempting to beat any system that exists. And even now there has at least been occasional legitimate cases. But in a more over arching notion, you're right that there's a big question of how much and what's it's impact.
In that regard, I can't honestly answer. It could be that the amount of Fraud is ridiculously miniscule and entirely insignificant. Or it could be a sizable portion that could have an impact. Or it could be somewhere in between.
Your comment about proof, and measurable numbers, speaks to my issue though. Essentially, as the laws and methods largely stand now, I don't think there are a lot of very efficient and sure fire ways to absolutely "prove" whether fraud is happening or not. Essentially, the way the system is currently set up, "proving" fraud is extremely difficult. As such, the lack of evidence of substantial fraud at the moment doesn't necessarily convince me or prove to me that fraud simply does not exist or does not exist in any meaningful way. It simply means that it doesn't exist in such a way that can be unquestionably proven under the current system.
To give you an analogy in regards to what I'm saying (though not a direct analogy to this specific situation)....
Say the only way I can "prove" something was stolen is to physically see a person gain possession and remove the item. Then say that I'm sat in front of a brick wall, with a bunch of large diamonds placed behind it. I see two people come up to the wall, disappear behind it, then reappear on the other side with a bag that has an approximately large diamond sized object in it.
Under the current criteria I'm operating under in terms of "proving" something, I can not "prove" that they stole the diamond. If I told someone we should take down that wall so I could see where the Diamonds are sitting to prevent theft, a person could accurately say "Why? There's no proven diamond thefts that are occuring". Technically, they're right...there's no proven diamond thefts occuring because the current structure of the rules make it so that it can't be proven. However, the fact it can't be proven under the given circumstances doesn't necessarily prove that it isn't happening.
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What I'm basically saying Boo is that you're right...we can't prove that voter fraud is happening at a significant level. I'm simply suggesting that those who claim there's "proof" that it's happening at an insiginificant level are not believable or credible in my opinion, because I don't believe the current system makes definitive "proof" of such far too difficult to obtain for it to be a meaningful statement to base a decision on either way.