Robert_Charlton - 8:51 am on Jun 6, 2010 (gmt 0)
...I am little confused between Google Adwords Keytool and Keyword Discovery Tool.
For a key phrase I was researching, adwords key tool was showing a global search volume of nearly 300,000 while Keyword Discovery was showing only 400.
Yes to everything that Receptional_Andy said about the Google tool... with an important addition about the Keyword Discovery Tool to answer the above question....
The Google Tool, with all its anomalies, is the only tool that gives anything that's remotely close to actual Google searches. It's got the largest and most reliable dataset, but it's not confined just to organic searches, and I think you've got to be careful about how you use it. If you're trying to get numbers for organic search, it skews data in a number of ways, as Andy describes... And be careful not to use it on Broad Match if you're targeting organic search.
WordTracker works from an extremely small sample size, and IMO is completely unreliable for infrequently-searched terms.
Regarding the difference in Google and Keyword Discovery numbers that you mention, note that the (Trellian) Keyword Discovery Tool is not purporting to give monthly search volume. It gives an accumulation of data over time, which is helpful for establishing relative search volume, but not monthly volume. I find it more granular than WordTracker, but it's still got a smaller dataset than Google. It's got its own set of limitations (noted in threads below), but, among free tools, I find the free version of the Keyword Discovery Tool second best to the Google Tool.
Other discussions on the subject, which compare the available tools...
Would you please suggest some useful keywords tools?
Google vs. Wordtracker - another one ;)