A new study from Efficient Frontier puts Microsoft at the top of the heap in terms of search ROI and says Yahoo gets more out of its partners than any other search engine.

The study, which found that Google improved across the board, highlights some of the key differences between the major search engines from an advertiser's perspective.

Yahoo, which has 1,196 content syndication partners, saw 55 percent of its paid clicks business come from those partners, compared to 45 percent generated in-house. However, the clicks generated on Yahoo's own site appear to have performed better, with Yahoo's in-house clicks accounting for 58 percent of all conversions.

In stark contrast to Yahoo, Microsoft, which has only five partners (all of which are MSN sites), managed to keep its ROI high by avoiding syndication. According to the study, MSN achieved high ROI for advertisers because it didn't serve search results on partner sites where a searcher's intent can be relatively low as compared to the main page.

But Google, which is still the undisputed leader in terms of total search traffic, seems to have found a balance. The search giant has 431 partners, roughly a third of what Yahoo boasts, but it managed to outperform Yahoo in terms of ROI, mostly because it had a high conversion rate for searches conducted on Google.com.

Overall, the study found that search ads served on syndicated partner sites simply don't perform as well, which is bad news for Yahoo because the company is more heavily vested in that model.

In the meantime, Yahoo, which has managed to move beyond the Microhoo mess, appears to be focused on transitioning to universal search with its new Glue product. However, the value of Glue will be severely undermined if Yahoo can't get more paid clicks in-house.