Webmasters often post questions online about the revenue potential of Google AdSense, and the answers vary all over the place.

It is against the terms of conditions for Google AdSense for any site publisher to disclose its click-through rates or other statistics about a site’s performance.

But enough information has been posted on the Internet to make some general observations about what to expect with CPMs (cost per thousand impressions), which are the result of the ad click-through rate and the revenue per click.

The first consideration is the business category of the Web site. It is well-known that certain categories attract strong advertiser interest, including travel, health, financial, real estate, employment, automotive and technology. Other categories do not perform as well, such as news, weather and sports. So anyone thinking about implementing AdSense or another contextual advertiser should consider the category first.

It follows that certain subcategories deliver a higher CPM than the overall category. A site that focuses on job listings might find that the health care subcategory outperforms the human resources subcategory. In that case, the webmaster might try implementing contextual advertising in just a subcategory rather than throughout the entire site.

In fact, anyone new to contextual advertising might want to experiment with it in just one channel of the site that has the greatest revenue potential. Sophisticated contextual advertising sites learn how to track performance by channel to determine if the ads deliver a higher CPM an alternative revenue source.

Finally, the implementation tactics have an impact as well. They include:

• Putting the ads near the top of the page;
• Placing them on the left side or center of the page rather than the right;
• Making the display similar to the surrounding content but just different enough that they don’t appear to trick the visitor into thinking they are actual content;
• Optimizing the surrounding content to attract the most relevant ads;
• Moderate the number of ads per page; too many will reduce the click rate.

Again, the results come down to click-through rates and revenue per click. And again, no site is allowed to share that information.

But other sites and webmasters who have posted such information claim that any click-through rate of less than 1 percent is underperforming, while anything over 5 percent is outstanding. A number of posts speculate that around 2-3 percent is average. It is this number that is most impacted by the implementation tactics.

The average revenue per click is more impacted by the categories (along with the economy and time of year) and varies greatly from one to the other, but various posts throughout the Internet suggest the average is well under $1. Certain highly targeted and effective sites might see a revenue per click above that number, while poor performers may be luck to average 10 cents a click.

These numbers lead to an average AdSense ad unit CPM or the more widely used eCPM, which measures the AdSense performance for an entire page based on the number of ad units on that page.

More ad units on a page increase the eCPM. A site that depends heavily on AdSense might consider a $10 eCPM as a benchmark for achieving above-average performance. Keep in mind that a $10 eCPM with two ad units per page equals a unit CPM of $5, while a $10 eCPM with three units per page equals a unit CPM of $3.33.