Nielsen//Netratings announced their May 2007 U.S. Search Share Rankings [PDF] today. Highlights include:

• Google: 56.3% share / 44.9% YOY growth
• Yahoo: 21.5% share / 18.6% YOY growth
• Live: 8.4% share / 0.8% YOY growth
• AOL: 5.3% share / 5.1% YOY growth
• Ask: 2.0% share / -2.8% growth

Last week Compete released their numbers and I suggested then that I’d like to compare figures between reporting firms. As I suspected, Nielsen and Compete came up with much different numbers. The most striking differences include:

• Google share differed by almost 11%; YOY growth differed by over 25%
• Yahoo share was similar across both services, but growth was wildly different. Nielsen said Yahoo’s YOY growth was 18.6% while Compete said Yahoo’s growth was -27.3%. That’s a difference of almost 46%.
• Both firms reported the same share percentages for MSN/Live.com. Again, however, the YOY growth numbers differed radically. Nielsen claimed Live.com grew by 0.8% while Compete said it dropped by 20.6%.
• Ask.com share percentages were close. But yet again, growth numbers varied by more than 30%.

So what’s the best approach to take to figuring this out? Should we average them together? Is one more accurate? Even with inaccurate counting, should the numbers be this radically different?

Combined Search Market Share
Compete + Nielsen//Netratings

• Google: 61.7% share | 32.2% growth
• Yahoo: 20.6% share | -8.7% growth
• Live: 8.4% share | -12.6% growth
• Ask.com: 2.75% share | -26.6% growth

If you combine and/or average stats from these two reports, that’s what you get. I’m not sure why… but something about those growth numbers just doesn’t seem right.

How do you decide which stats to go by? Do you have a favorite company? Do you average things out like this? How accurate are the numbers being spouted by the search engines themselves?