The best way to illustrate this is to take an example. With behavioral targeting, site visitation is a primary proxy for an individuals interest. For example, the following online behaviors may classify someone as an in-market car buyer or car enthusiast.
- Visitation to a car review site
- Configured a car with an auto online configuration tool
- visitation to http://www.audi.com
- User configured an Audi R8 on the Audi site
The point here is, if you are a data buyer for ad targeting purposes, demand your supplier to disclose on what criteria do they use for classifying their data, and further, at what incidence level should you expect these to occur with your life targeting. If you can't get a clear and quantifiable answer, I would suggest this raises some real question on what you are really buying.