How about =AVERAGE(other months) ± STDEV(other months)
But with such erratic data I would not bet my pay on the answer
best wishes
--
Bernard V Liengme
Microsoft Excel MVP
www.stfx.ca/people/bliengme
remove caps from email

"KCG" wrote in message
...
Hello all,

I am monitoring the performance of a machine. For each month listed below,
it broke down after running for the respective hours:

Month Hours
Jan-07 39
Feb-07 13
Mar-07 21
Apr-07 20
May-07 21
Jun-07 23
Jul-07 35
Aug-07 39
Sep-07 1
Oct-07 8
Nov 07 ?

For the month of November, using a Monte Carlo simulation (or similar) in
Excel, I want to predetermine the number hours that the machine will run
before it is next due for maintenance.

Thanx.

--
KCG