Google Analytics – Keyword Search “Not Provided” Keywords Estimator
You can use this simple tool to calculate the total number of non-brand visits or the amount of non-brand revenue including the data that is being hidden by “(not provided)” keywords.
http://www.digivate.com/not-provided-keywords-estimator/
This method used here can provide a good top level estimate. Another more indepth way to get the data is the landing page “clustering” method.
Calculating not provided keywords with great accuracy takes work. Below there are useful resources
www.seomoz.org/ugc/how-to-analyze-google-analytics-not-provided-data
searchenginewatch.com/article/2143123/How-to-Understand-Your-Google-Not-Provided-Traffic
www.blindfiveyearold.com/not-provided-keyword-in-google-analytics
cyrusshepard.com/7-fantastic-seo-tips-for-googles-not-provided-keywords/
www.barker.dj/blog/not-provided
www.bluefountainmedia.com/blog/making-peace-with-not-provided/
www.kaushik.net/avinash/google-secure-search-keyword-data-analysis/