keto - 2:31 pm on Jul 11, 2008 (gmt 0)
As someone that has had beta access to this feature for over a year now my tests show that the numbers are relative at best. I tested this by comparing a few hundred keywords that we always bid 1st in adwords to the number of searches that the tool shows. For terms that we averaged a position of 1.1 the impressions were about an average of 25% of what the Google tool was reporting for the total number of searches. After escalating this issue to our Google Adwords team at G they responded that the impression numbers where the single best way to determine search volume (if you bid an average of 1.5 or lower).