Author Topic: statistics question  (Read 1272 times)

statistics question
« on: 26 October, 2017, 12:06:19 pm »
Help me out.  I should know this but...

A baseline measurement is taken with a reading of 100 using metering rated at +/- 10% of the reading.
Following efficiency works, a post-retrofit reading of 80 is taken using the same metering (therefore same uncertainty).

What is the uncertainty in the demand reduction
( A ) 10%
( B ) 14%
( C ) 20%
( D ) 64%

What is your answer and why?

Re: statistics question
« Reply #1 on: 26 October, 2017, 12:17:24 pm »
Has the uncertainty of the metering measurement changed?  If not then your answer is within the text.

Re: statistics question
« Reply #2 on: 26 October, 2017, 12:31:55 pm »
That’s what I’d have thought, but the sample exam paper gives the answer as (D) which I can’t fathom.

Re: statistics question
« Reply #3 on: 26 October, 2017, 12:41:41 pm »
You have 100 +/- 10 and 80 +/- 8. When calculating the difference of two quantities the errors would typically be "added in quadrature", so the error is the square root of 10 squared plus 8 squared, i.e. root 164 or 12.8. The difference is thus 20 +/- 12.8 and 12.8 is 64% of 20.


Re: statistics question
« Reply #4 on: 26 October, 2017, 12:50:04 pm »
You have 100 +/- 10 and 80 +/- 8. When calculating the difference of two quantities the errors would typically be "added in quadrature", so the error is the square root of 10 squared plus 8 squared, i.e. root 164 or 12.8. The difference is thus 20 +/- 12.8 and 12.8 is 64% of 20.

Bingo! Thank you Philip.

I'd forgotten to calculate the standard error.  It's been a few years since I did my studies.  Hence the revision.  It's going well  ::-)