How does Google detect invalid clicks?

Detection and filtering techniques: Each click on an AdWords ad is examined by our system. Google looks at numerous data points for each click, including the IP address, the time of the click, any duplicate clicks, and various other click patterns. Our system then analyzes these factors to try to isolate and filter out potentially invalid clicks before they ever reach your account reports. This detection and filtering occurs over a number of levels including the following:
- Real-time systems filter out activity fitting a profile of invalid behavior (such as excessively repetitive clicks)
- Clicks and impressions from known sources of invalid activity are automatically discarded

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Advanced monitoring techniques: Various unique and innovative methods are applied at each stage of the filtering process, thereby maximizing proactive detection of invalid activity. Our engineers are also constantly improving our monitoring technology, enhancing filters, and examining a growing set of signals.

The Google Team: In addition to our automated click protection techniques, we have a team that uses specialized tools and techniques to examine individual instances of invalid clicks. When our system detects potentially invalid clicks, a member of this team examines the affected account to glean important data about the source of the potentially invalid clicks.

One of the goals of our team is to make invalid activity very difficult and unrewarding for unethical users, thereby decreasing their chance of success. Additionally, if we find that invalid clicks have been charged in the past two months, we'll credit advertisers' accounts.

Dr. Alexander Tuzhilin, an independent expert who has examined our detection methods, policies, practices, and procedures, has documented these and other details of our monitoring system. You can find his report here: http://googleblog.blogspot.com/pdf/Tuzhilin_Report.pdf (pages 25-26, 47). (English only)