If your map administrator has added custom statistical expressions, you will notice a Statistics drop-down menu on the top of the Results window after you have made a selection or performed a search. Select the Statistics drop-down menu and then select the option you want to run. The expression takes into account all of the selected results in the Results window.
If your map administrator has configured a statistical tool to run automatically after you perform a selection or search on a layer (group of map features), you will notice a dialog box appear displaying the title of the statistical tool and a numeric result.