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Difference?

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sunu:
I first found out about it looking at thousands of results and thought about rounding errors. But then I took data from ten tasks to look closely.

With 10 decimal points accuracy for the separate credit / runtime operations the difference is already 0.0014 between the two methods for only 10 tasks. I don't think it could be a rounding error.

Edit: Thanks for the link!

Jason G:
Have a look at the section on summing error & see which answer you get if you use your first equation using Kahan Summation or similar, minimising the division to the one final one.  That would be the 'most right' answer, though there isn't any 'right' answer in floating point... They're all wrong!  :o  :D

sunu:
Ok, I took 3400 tasks. Difference is 0.0017 almost equal with the 0.0014 from 10 tasks. This can't be a rounding error.

I'll look at Kahan Summation.

sunu:
I think an equivalent everyday example would be:

You drive from A to B and you want to know  your average km/h. This is elementary school stuff:  distance / time

The next time you drive from A to B you make 4-5 stops in between for coffee. How do you calculate your average speed now? Do you add the distance and the time and divide them ( sumx / sumy ) or do you calculate your average speed from each segment and then calculate the average as a whole ( avg (x / y))?

The last method now seems goofy but why is it right or wrong? And is the difference just a rounding error or avg (x / y) calculates something different?

perryjay:
On that second drive do you also have to figure in the restroom stops?   ::)

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