The NY Times has an article looking at the challenges faced by publishers and advertisers trying to determine web site traffic for advertising purposes.

As it stands today, many large ad buyers are relying on services like Nielsen to determine traffic numbers at large media sites. However, the numbers provided by one 3rd party service to the next can vary tremendously:

How Many Site Hits? Depends Who’s Counting

Other big media companies — including Time Warner, The Financial Times and The New York Times — are equally frustrated that their counts of Web visitors keep coming in vastly higher than those of the tracking companies. There are many reasons for the differences (such as how people who use the Web at home and at the office are counted), but the upshot is the same: the growth of online advertising is being stunted, industry executives say, because nobody can get the basic visitor counts straight.

Here are a few thoughts:

1. Media companies should provide better access to their site data so ad buyers can look at actual traffic numbers rather than relying on 3rd party surveys.

2. Media companies should use 3rd party tracking tools such as Google Analytics that can be shared with advertisers. While you may not wish to share ALL of the information available within Analytics with ad partners, the traffic information is valuable.

3. Ad buyers should learn how to understand the data generated by 3rd party stats companies, and understand the strengths and weaknesses of each system.

4. If ad buyers don't have confidence in a site's traffic numbers, buy pay per click rather than time or impression based ads.