Google Keyword Tool Accuracy (Review)

Recommendations

Based on the results, it is not recommended that you rely solely on Google's Keyword tool for predictions that require high accuracy and sensitive computations, such as sales. Customer expectations set using this raw data can be difficult to achieve, because of the high uncertainty of the predicted results when compared with the actual impression figures.

You can, however, use Google's Keyword tool as a “rough” guide or estimate, but it cannot be relied on by itself for making final financial decisions. In this case, it is recommended that you use the actual figures of search queries, such as those found in Google Webmaster Tools.

To provide much more accurate estimates, it is recommended that you use the “exact” match type in getting search volume figures that coincide closely with the actual impression data.

Finally, in keyword research, you can rely on Google's Keyword tool to determine those keywords that are good to target by simply looking at the search volume data. It cannot, however, be relied upon to provide accurate projections in terms of sales and actual traffic.

http://www.seochat.com/c/a/Google-Optimization-Help/Is-Googles-Keyword-Tool-Accurate-Analysis-Using-Actual-Data/1/