User response to paid listings varies by search engine.

A new study shows that Google and Yahoo! users find natural search results most relevant, MSN users prefer paid search advertising, and AOL users find both types of listings relevant.

Specifically, the Search Engine User Attitudes Survey, conducted by iProspect, found that:

• 60.5 percent of Google, Yahoo!, MSN and AOL users selected a natural search result over paid search advertisements as the most relevant on a sample query
• 60.8 percent of Yahoo! and 72.3 percent of Google search engine users chose a natural search result as the most relevant
• 71.2 percent of MSN users clicked on a paid search advertisement as the most relevant to their search
• AOL users identified both natural search results and paid search advertisements equally as often as the most relevant.

"These findings clearly indicate that in both search engines there is a wide gap between natural search preference and paid advertisement preference. This means marketers who fail to optimize for natural search or human-edited paid inclusion neglect a large percentage of user clicks and the relevant traffic," says Fredrick Marckini, iProspect CEO.

The study demonstrates that, in addition to making sure they show up in the natural (free) results of Google and Yahoo!, marketers should have a strong paid search presence on MSN -- at least for now.

"Users who may shy away from paid search listings when they see them may have difficulty distinguishing between the two types of results [on MSN]," Marckini says. But he adds: "It will be important for search engine marketers to track these metrics going forward, as Microsoft scraps its existing search service and replaces it with its new and improved search engine anticipated in the forth quarter. Because MSN’s new search service is rumored to be similar in look and feel to Google, it could change everything later this year."

As for AOL, the survey suggests marketers will achieve optimal results by dividing their efforts equally between natural search and paid advertisements.

"Taking a detailed look into how a particular search engine distinguishes between natural search results and paid search advertisements will allow online marketers to maximize visibility and increase traffic by tailoring their efforts to meet the users' 'preference' of each engine," concludes Marckini.