I am using the INDEX function twice [form:
AVERAGE(INDEX(lowerlimit):INDEX(Upperlimit))] to specify a sub-range of cells
within column, utilizing MATCH to find the row numbers of the lower and upper
limits. It has worked beautifully before, but, mysteriously, today I am
finding that the formula will give a #REF! error for any subrange larger than
two cells; that is, the second index must refer to the cell following the
first index, otherwise I get the error.

Can anyone give me a clue as to what the problem might be, so that the
formula can be fixed to return the expected value?

Thanks.