I have 13 data points by month. I need to calculate the rate of change over
the 13 months. Is calculating the percentage difference from month to month
starting with 03/08 and then average the results the correct way? Below is
the result I get, is this correct or is there an excel function specific for
this?

02/08 38
03/08 39 2.63%
04/08 41 5.13%
05/08 34 -17.07%
06/08 43 26.47%
07/08 39 -9.30%
08/08 38 -2.56%
09/08 32 -15.79%
10/08 38 18.75%
11/08 25 -34.21%
12/08 33 32.00%
01/09 30 -9.09%
02/09 24 -20.00%
03/09 29 20.83%
-0.17% (Average)

Many Thanks,

--
Mike