If you have a continuously running survey (Website/Email, etc.) you can measure trends in your data over time. Trend Analysis is basically calculating and comparing the results of your survey over time.
Go to : Login » Surveys » Analytics » Trend Analysis
From the Output/Report Data drop-down menu select the Detailed Statistics (Mean/Standard Dev./Confidence Interval) option.
See the screenshot below:
This feature is available with the following licenses :