Suppose all you data in column A from starting from A1, then in Column E
type formula

=AVERAGE(OFFSET($A$1,(ROW()-1)*12,0,12,1))

This will average every 12 rows, starting from A1.


--
Best regards,
---
Yongjun CHEN
www.XLDataSoft.com

=================================
"Raith" wrote in message
oups.com...
Hi, I just can't figure out how to do this so I'm hoping someone could
give me a little help.

I have a big list of measurements taken every 10 minutes.

Column A has the date/time
Column B has the integer value


What I would like to do is for each period of 2 hours (so it would be
12 rows) work out an average value and output that to a different
column.

So for example if I wanted an average over 30 minutes (intead of 2
hours just to make it simpler...

12:00 20 12:00 30
12:10 30 12:30 20
12:20 20 13:00 80
12:30 10
12:40 20
12:50 10
13:00 50
13:10 80
13:20 50

A is time, B is the value, and say column D is the start of the
averaged 30 minutes, and E the average. Hope that makes sense?