Originally Posted by ravo

In our experience, any traffic that opens in a new window/tab (eg. skimmed, popunders) Google Analytics will only count between 60%-70% of the total incoming traffic. So, what you are seeing isn't that unusual.

Your server logs should show very close to the actual amount of visitors sent, usually within 5%, and never more than 10% difference.

The advantage of skimmed traffic over, say, clicked targetted traffic is price. Yes, the quality will be lower, but if your target page is set up to be optimized for skimmed traffic it can be very effective, and a good value. That's why it's still being sold. (and that's why we have many clients buying $100's of skimmed traffic per day).

Google Analytics is great for determining the quality of the traffic from various sources. Use this info to create black/white-list of traffic. This is one of the keys to optimizing ANY type of traffic.

And, focus more on ROI than pure counts. If you spend $100, and made $150, that's good. If you only made $50, that's bad.