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[lazyfaire]

I think that might be called "percentage gain" not "rate of change".

If you're going to calculate a "rate" you would need a change in a second variable to which you compare the price change, like time, for example. Too bad I quit smoking cigars-- but just thought I'd chime in. :)

http://en.wikipedia.org/wiki/Percentage#Percent_increase_and_decrease

EDIT: rate of change

http://en.wikipedia.org/wiki/Rate_of_change_%28technical_analysis%29