I have a cell (AW4) into which one types a figure representing a previous
period's accumulated average. (The sheet is cleared after each period and
restarted)

I have another cell (AU4) which provides a current average across numerous
rows. (There are about 40 rows)

A third cell (AU1) shows a perpetual average from these two cells.

This formula, in cell AU1, is simple enough - =AVERAGE(AU4,AW4).

However, there are times when the user will not enter a figure in cell AW4.
When this happens the average shown in cell AU1 is only current and not a
perpetual reading, which can be misleading, so in this event, I want AU1 to
remain blank. In other words. if nothing is entered in cell AW4 how do I
adjust the formula to prevent a figure (from only AU4) appearing in cell
AU1?

TIA for any help.

Ed