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Topic: Matlab - RMS-value
Replies: 2   Last Post: Oct 11, 2012 11:16 AM

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 Tom Lane Posts: 858 Registered: 12/7/04
Re: Matlab - RMS-value
Posted: Oct 11, 2012 10:08 AM

> RMS1 = sqrt( mean(inVec)^2 + var(inVec) );
> RMS2 = sqrt( 1/length(inVec) * sum(inVec.^2) );

...
> Why is there a difference ? Round-off error or something else ?

You're right, something else. Ordinarily var uses 1/(n-1) rather than 1/n in
estimating the variance of a sample of size n. You can supply a flag to get
it to use 1/n, and then the results match:

>> v = 1:10;
>> sqrt(mean(v)^2 + var(v))

ans =
6.2783
>> sqrt(1/length(v) * sum(v.^2))
ans =
6.2048
>> sqrt(mean(v)^2 + var(v,1))
ans =
6.2048

-- Tom

Date Subject Author
10/11/12 text-dude
10/11/12 Tom Lane
10/11/12 Richard Crozier