Quoted:
I'm having a brain dump and can't figure out the correct method for figuring out % difference/change. In my case I'm trying to determine the correct percentage difference reading of some blood glucose measurements. Comparing a CGM(continuous glucose monitor) sensor with a blood sample test. The CGM is reading 75 while the blood strip is measuring 107. The sensor has a 20% guarantee. I have a phone app I use for figuring % calculations. Do you do this calculation looking for a percentage change or a percentage difference? Seems they are different numbers.
But in my case I need to know if the CGM sensor reading of 75 is above or below the 20% margin of error when compared to blood strip reading of 107.
If I use my phone app and enter "From value:" of 75 "To value:" of 107 I get 38.67. However if I reverse those numbers I get -27.88
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Both numbers you have are correct. 107 is 38.67% greater than 75 (because you are using 75 as your base measurement), and 75 is 27.88% less than 107, because you are using 107 as your basis, and a given difference will be a smaller percentage of a larger number (using a larger number will also result in a negative change, since the value is decreasing, though difference is often not considered to show direction of change, and will therefore be represented as a positive number no matter which measurement is larger). Difference and change are the same, what changes the results are what you use as your starting measurement. For an example which is both simple and obvious, let's say your two measurements were 100 and 200. 200 is 100% larger than 100, but 100 is NOT 100% smaller than 200. Even more drastically, 0 is 100% less than ANY positive number, but no non-zero number can be measured as a percentage change FROM zero, because changing zero by any percent gives you zero.
There are two takeaways here:
First, neither way of measuring puts your continuous measurement within 20% of the strip. Second, that does not necessarily mean that your monitor is not within 20% of the actual blood glucose level, unless you know the accuracy of the strip test.
Mike