Web analytics
First there were web analytics companies that offer a software-based solution which resides on web servers (generally UNIX-based). These web servers do measure hits.
A hit is an entry in the log file of a web server. A hit is generated by every request made to a web server. It has no predictable relation to users, visitors or pages. For example, a page loading on a browser produces a hit. And every individual piece of graphic that loads on a browser produces a hit. Therefore, a page with 24 graphics on it can produce 25 hits. In the early days, we used to joke that if a seller tried to prove how big his site was by telling us how many hits it had, we would throw the seller out and tell him to come back when he had a clue.
The web analytics programs can also measure pages viewed (pageviews), visitors, unique visitors and a number of other metrics. They generally do this for any visitor from anywhere on this planet (not sure about ET visitors). So, they are not parsed for United States or any other countries' traffic, which is what many advertisers are looking for. In addition, they generally cannot tell whether a human or a machine is causing the visit. Thus, they are not generally filtering out for bots or spiders, etc. In many cases, the construction of the website involves the launching of many daughter pages, thus artificially inflating the number of pageviews that are produced.
There are many other factors involved but the bottom line is that these are webmaster-oriented tools and were not constructed to either measure advertising or even work with the tools that do measure advertising. This is a huge problem that we'll deal with in the upcoming solutions section.
A bigger problem is in the positing by the publisher sites that the numbers produced by the web analytics software represent the "real numbers." Jim Sterne, head of the Web Analytics Association, has gone on record that if a site were to use three different web analytics programs, it would get three different numbers. More to come on this...