It is well established that it is impossible for any web analytics technology to be 100 percent accurate. Most systems are accurate to within 95 percent.

I have asked Google how accurate they consider the system to be. So far they have declined to provide any answer, including claiming having claimed 100 percent accuracy. Let's give Google the benefit of the doubt and consider the possibility that AdWords' click count is 99 percent accurate. This would make it the most accurate web analytics technology on earth by a long stretch. It would also mean it had mis-billed over $60 million last year. If it was 95 percent accurate, then it mis-billed $285 million last year.

My personal experience is that the accuracy of the billing system in Google AdWords varies. I run PPC campaigns for clients, and I always check Google's numbers with what our own software, InSite, says. Since we always run a unique tracking code on every Google ad, this is fairly easy. I have disputed numbers with Google several times over the last few years.

I'm not talking about click fraud here. Click fraud disputes do not involve a disagreement about the volume of traffic Google has relayed to your site. Click-fraud involves a disagreement about the validity of that traffic. Both sides agree on the numbers; they just disagree on whether all of it should be paid for.

The situations I'm concerned with are where Google AdWords claim to have sent more visitors to my site than I can verify for myself.

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