The Engine Ready project found that the cost per acquisition was most favorable at the second ad position, where CPA was $38. By contrast, acquisition costs averaged $60 for the first, third and fourth ad positions in the campaigns examined and actually rose to $84 for slot 5.

Interestingly, the second best CPA in the project was posted by ad position 7, where it cost on average $50 to get a customer to do something. "No one's really managing for ad impressions now because PPC impressions don't cost anything," Lewis says. "But it may be worth looking at how effectively impressions in different ad positions lead to conversions, because those impressions are the starting point for any sales."

Finally, Lewis thought it would be productive to take a look at a metric not often examined in search analytics: conversions per 10,000 impressions, to get a sense of the relationship between the number of times an ad is served in a particular position and its power to drive actions among viewers.

As it turns out, the fourth ad slot is powerful here, producing on average 12 conversions for every 10,000 views for Engine Ready's clients. That was significantly higher than any other ad positions tracked; the next contender was the Number Seven spot, with 7.6 conversions per 10,000 views, followed by position 1, where the conversion/impression rate was 5.5 in 10,000.

Rather than turning up broad rules that can be applied to any marketer's PPC ad positioning across any industry, Lewis says Engine Ready's inspection of its compiled client data suggests the importance of understanding the unique "PPC personality" of specific industries and markets. The kind of products and services being marketing can affect the significance of position, as can the buying behavior of prospects, strategies used by competitors, the seasonality of a given industry and marketers' determination to own a specific keyword for branding purposes. And of course conversions depend on a lot more factors than just ad position, he points out-including optimized headlines, creative, and landing pages.

But Lewis thinks the Engine Ready client data, informal and selective though it may be, points to the possibility that PPC sweet spots live at the fourth and seventh ad positions, where optimum conversion rates tend to produce optimal return on investment.

"We intend to look at this data again in six months, and we'll be watching for indications that show those same trends," he says.