# Calculating increased risk (% increase) Watch

1. Stupid question,but I can t get my head round this .
See attached doc.
I do not understand how people know which value to divide by.

Calculations: 420-30 : 390
Then you have to divide it by 30 to get the right answer,but why 30 and not 420 & multiply by 100% to get percentage?
Is it that you always have to work from the left to the right,surely there is a better explanation?
There is no original value & no change in time,so those formulas don t work.
Attached Files
2. Calculating increased risk (% increase).docx (74.7 KB, 676 views)
3. (Original post by hissyfit)

Calculations: 420-30 : 390
Then you have to divide it by 30 to get the right answer,but why 30 and not 420 & multiply by 100% to get percentage?
Is it that you always have to work from the left to the right,surely there is a better explanation?
Because you want to work out how much greater risk a 75 yr old is at compared to 18-44yr old so you find the difference (390 for every 1000 people) and then you want to know how much greater this difference is compared to the 18-44 value.
(i.e. how many times does 30 go into 390 or how many more times at risk is a 75yr old person).

Does that make sense? :-)

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Updated: December 13, 2010
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