SUM is just one of the many Excel functions. These prewritten formulas simplify the process of entering calculations. Using functions, you can easily and quickly create formulas that might be difficult to build for yourself.
You can use the AVERAGE function to find the average absolute deviation from the median.
Excel will enter the formula for you. Click in cell E102. On the Home tab, in the Editing group, click the arrow on the Sum button, and click Average in the list. The formula =AVERAGE (D2:D101) appears in the formula bar 
near the top of the worksheet. Excel will tend to use the closest cell with a numeric value in it in these functions. If you placed the cursor at E103, Excel would attempt to use D103 as an argument. If you want the average to appear in E103, you should type the formula in that cell instead of using the menu.
The coefficient of dispersion can be easily calculated using the previous results. Move your cursor to an empty cell, say F102 and enter the formula '=E102/D103'.