This invention is related to web-traffic and used in online advertising including interactive TV, cellular telephone or Personal Digital Assistant PDA. It discloses a system to detect invalid and fraudulent impressions and clicks and method of pay-per-click (when advertisers pay upon users actually clicking) and pay-per-impression (when advertisers pay based on number of views) advertising arrangements, which periodically generates a code associated with the search-engine users. This code, preferably in the form of a serial number, is compared to the user of the website, such that by observing a metric like the number of clicks for a given period of time, be it a short time or a longer period, such as a day or a week, the system can automatically determine if certain clicks are illegitimate. This allows the search engine company to fairly invoice the merchants, thereby preventing fraudulent over use.