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#1 2005-08-15 14:38:05
- Luca
- Guest
percent difference between 2 zero values
Hi, I'm a software developer currently working on a statistics program. I have a series of values per month, and for each month I have to calcultate the percent difference from the preceeding month. Example:
January = 50; February = 75
% difference between January and February = +50%
I calculate this as follows:
[(75 - 50)/50]*100
Everything works fine until get 2 consecutive months with a 0 value, example:
March = 0; April = 0
% difference between March and April:
[(0 - 0)/0]*100 => which of course is infinite and returns an error. But we all know that in fact the % difference between these 2 months is zero...unfortunately a computer doesn't understand this.
My question: is there another formula I can use to calculate the difference, that will not return an error when 2 consecutive values are zero?
Thank you very much if you can help me.
#2 2005-08-15 16:04:53
- ganesh
- Moderator
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Re: percent difference between 2 zero values
If the value of the current month is B and the previous month A,
[(B-A)/A]*100 is the right way of calculating the percentage increase, which you have done. Problem arises when the difference is 0 and both the values are 0. I don't think there is any mathematical solution, but if you could program in such a way that
the formula is applied only when the difference ≠ 0
and when the difference is zero, the percentage change is zero,
your problem could be solved!
Character is who you are when no one is looking.
#3 2005-08-15 16:44:01
- MathsIsFun
- Administrator
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Re: percent difference between 2 zero values
Yes, this comes under the heading of trapping exceptions, in this case the divide-by-zero exception.
You would have the same problem if one month was 0 and the next anything. For example 0 to 10: (10-0)/0
It is because you are asking "what is the relative change since last month" - how do you relate to a zero month?
So, you must have an "if (A == 0) { PctDiff=0 } else { PctDiff=((B-A)/A)*100 }" kind of code. (I just made PctDiff=0, but even better would be to print out an "N/A" or something to show that you know it is not possible to calculate the %)
BTW, what language are you writing it in?
"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman
