So here is what I'm looking at...

December - 100 - N/A
January - 200 - 100% change (1.0)
February - 300 - 50% change (.5)
March - 100 - 66% change (.66)

I can tell what the percentage change is from month to month, but I'm
not sure what the best way is to figure out the average percentage
change, overall. I can't add up the decimal versions of each
percentage difference and then divide by the number of months to get
some kind of overall average, right? In this case, that would turn
out to be an average variance of 72%.

Please help - thanks!