waterfall chart - math

[Anonymous]

Ok, so trying to wrap my head around this math.   I understand how the waterfall chart works in Power Bi in terms of adding/subtracting between periods.    

What I don't get is the math.   This chart is measuring the number of product returns we had from one month to the next.   Below is what the tool tips say (for the two big red bars).  

I do not understand the math of why is an absolute change of 36 an increase of 400%, but a change of 31 is only 69% ??

The math checks out...I've use a percentage calculator to check it....but my mind has still not wrapped around this yet.   Any math folks out there that can explain this?    I have to try and explain this waterfall chart to folks that understand it even less than I do...and when I show them this I know I will get questions.  

Return Item   abc                                                Return Item   abc

May Return Qty      9                                          June Return Qty     45

June Return Qty     45                                         July Return Qty      76

Qty Change           36  (400%)                             Qty Change           31 (68.9%)

[Anonymous · Accepted solution]

The maths is correct. 

If you start with 9, then 9=100% of your starting quantity 

9+9+9+9 = 100%+100%+100%+100% = 36 = 400% of 9

100/9 x 45 = 500% = Therefor, a 400% increase. 

If you start with 45, then 45 = 100% of your starting quantity 

100/45  x 76 = 168% = Therefor, a 68% increase. 

Think of it this way, 

You have 2 apples. Therefore, 100% of the apples that you have = 2. right? 

Now, you are given 4 more apples. If your original quantity was 2 but now it's 6, you've got three times the apples, therefore, 3 times 100% = 300% . So the change would be 200% (because you started with 100% (300% - 100% = 200%)