
Elementary Math Fractions, Percentages, Word Problems, Equations, Inequations, Factorization, Expansion 
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July 30th, 2015, 07:17 PM  #1 
Member Joined: May 2015 From: USA Posts: 34 Thanks: 1  Percent Change 
July 31st, 2015, 02:06 AM  #2 
Senior Member Joined: Apr 2014 From: Glasgow Posts: 2,130 Thanks: 716 Math Focus: Physics, mathematical modelling, numerical and computational solutions 
There are few mistakes here, but nothing too major: Firstly, the final column, should be renamed to "% change in proportion" to make it clear to the reader that that is what you are calculating. Most people would think of percentage change as "% change in number of units". I would add another column at the end to represent this new percentage so that you have both in your table. Secondly, the "% change in proportion" of the total appliances is actually 0% because it will always be 100% of the total. This figure isn't particularly useful! The 8% you have calculated is actually a "% change in number of units". Thirdly, percentage changes between different appliances won't add up, so you cannot total up "% change in proportion" to get a "total % change in proportion". When doing accounting, try to keep everything in terms of raw data (such as the number of appliances) and get totals straight from the raw data instead. Then, after that, percentages can be used to characterize that data and provide insight as to what the numbers represent. 
July 31st, 2015, 04:01 AM  #3 
Member Joined: May 2015 From: USA Posts: 34 Thanks: 1  
July 31st, 2015, 07:04 AM  #4  
Senior Member Joined: Apr 2014 From: Glasgow Posts: 2,130 Thanks: 716 Math Focus: Physics, mathematical modelling, numerical and computational solutions  Quote:
Let's say we have two flavours of ice cream, Vanilla and Strawberry. We have 10 tubs in total and an initial proportion of 50% for each (so we have 5 tubs of each flavour). Now we replace a Vanilla tub with a Strawberry one so that there are 4 Vanilla tubs and 6 Strawberry tubs. The proportion is now 40% Vanilla and 60% Strawberry. The change in proportion is 10% for Vanilla and 10% for Strawberry. The "% change in proportion" is 20% for Vanilla and +20% for Strawberry. So what is "total % change in proportion"? Well, we add up 20% and 20% to get 0%. If we are only allowed to replace tubs with different flavours, then this parameter is always going to be 0%. The only way to get it to change is to be able to change the total number of items. Therefore, I would suggest that "total % change in proportion" is not a useful parameter at all. If you want to get an idea for how much variety there is in your stock and how this changes with time, then I would recommend calculating a different parameter, such as the rootmeansquare (rms for short). To do this, 1. take your "% change in proportion" results 2. square each number 3. calculate the average 4. square root the final result. For example, If we square the 20% for Vanilla and +20% for Strawberry, we get 400% and 400%. The mean of those is (400%+400%)/2 = 400%. The square root of that is 20%. Therefore, the rms "% change in proportion" is 20%. The advantage of rms measures is that they are simple to do and they work for any number of values. Try calculating it for your data sample Last edited by Benit13; July 31st, 2015 at 07:07 AM.  
July 31st, 2015, 10:49 AM  #5 
Member Joined: Jun 2015 From: Casablanca Posts: 47 Thanks: 3 
If you aren't asked to use rms, then following your method: $\left(\dfrac{510}{100}\right)*100=50 \% $ $\left(\dfrac{4020}{20}\right)*100=100 \% $ $\left(\dfrac{2030}{30}\right)*100=33.33 \% $ $\left(\dfrac{6560}{60}\right)*100=8.33 \% $ 

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