Writing a macro

[RB]

Hello,
I have a workbook with 12 worksheets that refer to sales data for each
of the months for 3 years.

I need to calculate the average and std. deviation for each of months
for each of the three years. It's rather tedious to do this for each
month of each year.

How do I write a macro to make this easy for me so it will do it for
all the months?
Each worksheet has sales data for Jan 2005, Jan 2006, Jan 2007
So there are 12 of them for each of the months.

Can somebody help me with this problem?

Thanks,

RB