Unfortunately as the above example shows there are a few inherent weaknesses and pitfalls with using the Overture Search Term Suggestion Tool. You can also read the review of the Google AdWords keyword suggestion tool that has other limitations.
In summary, Overture do some "conversions" to the most popular terms that they display. Such as, misspellings, hyphenated terms, punctuation variations, singular and plural combinations are "merged", "combined" and "mapped" to the most common form of that term.
From the above example:
As a result the Overture data can be over inflated and inaccurate for specific search phrases. At the same time other factors even further inflate the search counts such as web site rank checking apps.
Why does Overture do this? All of these features are great for Overture PPC content match advertisers, in short Overture simplified their keyword tool for use by their PPC advertisers, in order for them to see the potential search volumes they can expect without having to target every single keyword variation. So if you are running PPC campaigns on Yahoo, then the tool shows the number of impressions you will get for your ad. Not all are human generated, but nonetheless still count as an ad impression.
The problem with this is that for organic SEO or even PPC campaigns on any other search engine like Google, this data can be rather unreliable. That is why in terms of SEO, use the Overture data wisely and with the above in mind. This is where cross referencing with an alternative keyword tool that does not merge plurals and like meaning phrases is very much recommended.
For example checking the Premium keyword database in KeywordDiscovery you can quickly tell which phrases are actually searches and in what frequency to all the others.
One other point to consider when using the Overture keyword tool, is that the search counts are from a Single Month, (January 2007) which does not take into account seasonality factors at all. Simply there is no point doing research for valentines day in November as Overture will have very little search activity on that seasonal topic for most of the year. You will need to do research in Jan, Feb and Mar in Overture to get the data you need for Dec, Jan and Feb.
As KeywordDiscovery provides a full 12 months worth of seasonal data it does not matter when you a doing your keyword research, the keyword will be represented over the last 12 months and broken down to weekly and monthly search counts.
Overture have ceased updating their database, it will only return data for the month of January 2007.