Assuming that you are using a table to store the 'last 14' entries, then use the;
TABLE/FORMULA menu item to put a formula in the 15th cell of the particular column in the table.
From the dialogue box you can enter the formula you may need to use similar to that done in Excel; i.e. AVERAGE(A1:A14).
Otherwise, why not set up a speadsheet in Excel and CUT/PASTE it into your MS Word document.
Or you could use the INSERT/OBJECT menu item to insert a (fresh) Excel worksheet into your document.