A leading Web analytics firm has added a new calculation method to its monthly search engine figures in the face of controversy over how it defines searches. The question, however, is how that affects the numbers that search engine optimizers and Web advertisers use to determine where to spend their dollars each month.

The difference is an important one, and one that advertisers are not likely to sort out immediately.

The firm, comScore (NASDAQ: SCOR), has come under criticism from some analysts in recent months, for counting "slide show" features introduced by Microsoft's (NASDAQ: MSFT) Bing and Yahoo (NASDAQ: YHOO) that change as the user scrolls over them as valid searches -- even though the user may have had no intention to perform a search.

This month, comScore introduced what it calls "explicit core search" figures alongside what it refers to as standard "total core searches" -- the latter includes the slide show searches while the former does not.

"Going forward, we will include in our public release of the monthly U.S. search data both the traditional Core Search and Explicit Core Search views of the market [which] will empower each interested stakeholder to determine which view of the market they deem most appropriate depending on the particular circumstance," Cameron Meierhoefer, an executive vice president of analytics at comScore, said in a post to the company's blog Monday.

So how do the changes affect comScore's measurements for July?

Using the new explicit core search calculations, Google search lost 0.4 percent from June's figures to bring its July position in at 65.8 percent of U.S. searches -- down from 66.2 percent in June.

However, in June, comScore had not yet changed its modeling method, so total core searches was the only calculation method at that time. Using total core searches, Google's share fell a point in July, from 62.6 percent in June to 61.6 percent share. The influence from the slide show features seems to have skewed the data in favor of Google's competitors.

Similarly, Yahoo's user share in June, using the new explicit core calculations, picked up 0.4 percentage points in July to reach 17.1 percent -- that's up from 16.7 percent in June.

Measuring by total core searches, however, Yahoo came in at 20.1 percent in July, a gain of 1.2 percentage points from June's figure, which was 18.9 percent.

Meanwhile, Microsoft's explicit core search numbers had Bing remaining flat month-to-month with 11 percent user share. In contrast, total core search numbers gave Microsoft 12.6 percent of U.S. searches, a tiny drop of 0.1 percentage points from June's 12.7 percent user share.

Stuart J. Johnston is a contributing writer at InternetNews.com, the news service of Internet.com, the network for technology professionals. Follow him on Twitter @stuartj1000.