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According to a representative from Ask.com on a panel at SMX West in February 2009, the average query length for searches on Ask.com is 3.89 words per query. 44% of all their queries are 4 words or longer. 

Queries of 5 words or more are up 9% since 2008 and queries of more than 8 words have increased 20% since last year.

This demonstrates users are becoming more sophisticated in the way they do searches and it validates the case for buying long tail keywords in SEM campaigns.

If you need help finding long-tail keywords for your ads, check out Google's new Search-based Keyword Tool.