This seems a perfect application for a Pivot Table.

http://peltiertech.com/Excel/Pivots/pivotstart.htm

If you make your data field the Average of diameters (rather than the
Sum), the PT will by default average by second, regardless of the number
of data points in each second.


In article ,
GMB_Excel
wrote:

1. Is there any way to average for every so many rows? If yes, is there
a way to quickly have an automatic check to see if every second has 25
rows to it? If I cannot is there a way I can work around the
possibility of having a few seconds that will contain a different
number of rows?