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Percentage Change or Percentage difference

  • 16-02-2013 6:59pm
    #1
    Outkast_IRE
    Registered Users, Registered Users 2 Posts: 7,516 ✭✭✭


    I am writing up an experiment at the moment , bascially calculating energy/head loss across various components in a fluid system.

    There was a manometer from which actual head loss reading were taken.
    So these values are the actual measured head loss.

    I then had to work out the "theoretical head loss" which i have no problem.

    There is obviously a difference between the measured lossses and theoretical losses .

    When comparing these figures i am tempted to use "percentage difference " to show the difference but most people would naturally use "percentage change".

    For example one of the actual readings was 287mm and theoretical 408mm using the percentage difference method it says its a 34.8% difference .

    Just wondering what the correct way to show the % difference between the values is ?


Comments

  • #2
    jeepers101
    Registered Users, Registered Users 2 Posts: 708 ✭✭✭jeepers101


    Where did you get the 34% from? Could you describe your actual result as being a certain percentage lower than the theoretical result?


  • #3
    Outkast_IRE
    Registered Users, Registered Users 2 Posts: 7,516 ✭✭✭Outkast_IRE


    jeepers101 wrote: »
    Where did you get the 34% from? Could you describe your actual result as being a certain percentage lower than the theoretical result?
    This is the way percentage difference works

    Calculate percentage difference
    between V1 = 287 and V2 = 408

    ( | V1 - V2 | / ((V1 + V2)/2) ) * 100

    = ( | 287 - 408 | / ((287 + 408)/2) ) * 100
    = ( | -121 | / (695/2) ) * 100
    = ( 121 / 347.5 ) * 100
    = 0.348201 * 100

    = 34.8201% difference

    Obviously its a far different result if you talk about percentage change, but im reading up on it and trying to understand when to use each way.........

    it seems from my reading that percentage change is fine to use in a situation such as in 2010 my house was worth 100,000 , in 2012 my house is worth 110,000 the percentage change is x amount .

    But percentage difference is used when there is no direction of change such as in this case............. one is a theoretical value and the other is a reading. There is no "change" .

    Im thinking % difference is the right way to do it in this situation but i may need to put a note where i calculate it to justify my decision , or maybe mention it in the discussion of the write up .


  • #4
    Carawaystick
    Registered Users, Registered Users 2 Posts: 8,779 ✭✭✭Carawaystick


    Without thinking about the numbers or even too much about the titles.
    you are presenting the difference between theoretical and measured values, so it's a difference.

    You are probably worrying about the theoretical being y and the measured being x and is it y/x or x/y

    I'd always set the theoretical value to the baseline and have the measured value as the difference to this.


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