Data Analysis Tookpak

Recommended for advanced Excel users, the Data Analysis Toolpak is a free add-on for Excel which enables the processing and interpretation of complex data, statistics and engineering calculations through a range, or ‘Toolpak’ of additional functions, including Correlation, Rank and Percentile, Random Number Generation and z-tests for variance. You can activate the Toolpak through Excel by clicking File>Options>Add-Ins, where you can choose to install the Toolpak if you haven’t already.

http://office.microsoft.com/en-gb/excel-help/load-the-analysis-toolpak-HP001127724.aspx