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My advertising system sells advertising on a CPC (cost per click) basis, not CPM (cost per thousand impressions).

It is therefore very important that my clients' clicks aren't being "used up" by malicious users clicking ads over and over, or by programs like spiders, crawlers and robots who visit the site and click on every link in sight!

These methods do work - here's a quote from Gavin Williams at - a leading component supplier:

"Your click-through counting has been much more accurate than other ASP sites on which we've advertised. They've overcounted by as much as 100% to 300% due to spider click-throughs. I'm impressed that your spider-filtering has kept your counts within just a few clicks of ours."

...and that was before I added more code to cope with some other common "non-human" clickthroughs. You'll see all of these methods in the next few pages...

Part 2: One clickthrough per IP per day...