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Hi Florent, For me if Webmaster tools shows me 1600 impressions and I'm on the first page, that means 1600 people searched for the term, I consider that search volume, this doesn't apply to search results where I'm not on he first page. A 2.5 position can be real even if you are no1 for 6 weeks. The reason being is personalized results, you can be no1 on your results and be no 3,4 on other people results. Are you sure that 15.000 searches are 'exact match' searches?