you have a couple options.. Pivot table or sumproduct.

For you, pivot table may be better..

highlight the table, go to Data - Pivot Table........

In the row field, put, say, the date column.

In the column field, put the name from column E of your table.

In the Data field, put the value from column F.

So, each cell of your table will be the interestion of the company name and
the month of service.

Right-click on one of the cell in the table, click field settings and choose
average.

Click the Number... button to format however you would like.

HTH!

"SusanU" wrote:

I have a list of data (about 19000 rows/data values) with water quality
values at 53 locations ranging from March, 1979 through March, 2003. Some
months have 1 value, others have anywhere from 2 to 4. I need helping coming
up with a formula that will calculate the average, and that I can copy to the
cell below, for example the average for April, 2001 in cell K250, and the
average for May, 2001 in cell K251.

D E F (value)
465 Apr-01 Salinity 33.5
466 Apr-01 Salinity 29.6
467 Apr-01 Salinity 27.8
468 May-01 Salinity 32.6
469 May-01 Salinity 32.5
470 May-01 Salinity 32.3

Also, I seem to have a hard time grasping how the more complicated formulas
and arrays work. In spite of hours of reading and trying to understand, it
isn't intuitive to me. Please bear that in mind when you answer my question!
Thank you!!