If you do not want some answer items to be included in the mean calculation (such as N/A in a continuous choice question or the "Other" option in a question) then un-check the box corresponding to the answer and the analysis engine will disregard that answer in calculating the mean.

Where can I change this option?

This option can be enabled/disabled by going to:

• Login »  My Surveys »  Reports »  Advanced Analysis »  Configure Analytics

Screenshot

You can deselect the options that you do not want to include in the mean calculations and save the changes by clicking on the update button.