Im doing some research on some keywords and every tool I use gives me way different results: Wordtracker says my phrase gets searched for 150/day Overture says 1,300/day SEO books tool says nearly 5,000/day lol, very frustrating. Which do I go by? this is a huge gap between the three.
Login your Adwords account and setup a campaign for your keyword(s) and set high bids and test it for sometime {min 2 weeks} then you will have click data which is about 10% of total traffic on Google for you keyword(s)
I normally go by the Overture results. I don't take the number as an exact result but rather use the numbers to judge the keyword in relation to others. I also take the fact that if 10000 people may search on Yahoo, then 30000 people may search on Google.
The numbers are for different search engines, so it's not strange they are different. From google and yahoo's yearly top keywords, you can see that they are completely different. The number is just an indication, it's not the exact number for searhes in google. You'd better take all 3 into account.