I use a spreadsheet to record test scores. There are 12 tests to be taken
over the year and I would like to calculate a running mean of the scores as
they are inputted. I have tried just calculating a mean but the spreadsheet
does not appear to be able to cope with calculating a mean from a formula
which refers to empty cells (those cells referring to the tests which have
yet to be taken).I am using Excel 2003 and I don't have the statistics
package available.

How do I calculate a running mean?