is there a standard or most correct way to calculate
I have always calculated percentages in the following way:
what is the percent change, when the temperature (for example) rises from
100 to 101
((101-100)/100 )x 100% = 1% or when is the percent change when the
temperature falls from 101 to 100, ((100-101)/101) x 100% = 0.99%.
The way a percentage change is determined is standard and is:
% change =100 x ((new value-original value)/original value).
You can always avoid the confusion by carefully state the reference point
(= original value). For example, saying that "the population of California
has increased 8% in the past ten years" is quite unambiguous.
Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory
Argonne, IL 60439
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Update: June 2012