For each widget in your dashboard you can select what type of calculation that you want to apply to your data in order to present the result. 

If you set your Datapoint Type to Dynamic, you will be presented with a list of possible calculations under Datapoint Main Type (see image below).

Please note: As a portal administrator you are able to predefine the list of calculation options that your users can choose from. You can manage this list within the Reporting KPI & Calculations tab of your portal management.