ClickFacts, Click Forensics and AdWatcher are all feeling the heat this afternoon as Google fights back against these firms reports regarding the degree of click fraud.

Google is essentially claiming that the studies are either based on guesswork or they are tracking too many "fictitious clicks". The PDF of the report is available on the Official Google Blog (PDF here) and another post about these "troubling findings is available at the Inside Adwords Blog.

Both are interesting reads to get 1) some background on third-party click fraud estimates and methodology problems, 2) valuable findings from Google's internal click fraud auditing team and 3) recommendations for addressing the issues. There are also demonstrations of how fictitious clicks occur in third-party systems and some very detailed case studies. A must read for any one concerned with invalid clicks from pay-per-click advertising.