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#1 2005-08-14 16:38:05

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 2005-08-14 18:04:53

ganesh
Moderator
Registered: 2005-06-28
Posts: 12,940

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.

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#3 2005-08-14 18:44:01

MathsIsFun
Administrator
Registered: 2005-01-21
Posts: 7,529

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

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#4 2005-08-16 13:22:04

ryos
Member
Registered: 2005-08-04
Posts: 394

Re: percent difference between 2 zero values

I like it like this:

percentDiff = ( (A == 0) || (B - A == 0) ? 0 : ((B - A) / A) * 100) )

But everone has their own style. wink


El que pega primero pega dos veces.

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