You can use the percent difference formula when you want to know the difference between two values in terms of percentage. It is used to calculate the percent difference between two values of the same category or class. For example - calculating the percent difference of budget increase from one year to the next, or the population growth over a period of 10 years. Scientists often use percent difference while comparing two observed values of a particular experiment.
Read more at Buzzle: http://www.buzzle.com/articles/calculate-percent-difference.html