Quote from Swish:
The problem w/ averaging is that is doesn't accout for the order of the returns.
For example, if you lost 50% in year one, the 20% gain in the next year should be based on the adjusted capital basis at the beginning of year 2 - thus a 20% gain in year 2 only recovers 10% or 1/5 of your loss in year 1.
Thus, I recommend that you convert the %'s to $ and calculate an rate of return based on the irr function in excel.
Swish