How do i add normal error to some ideal values ?

2012 6 2
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Hello,
I have a set of ideal -error free- measurements and i want to add some error to them to simulate measurement error (noise etc). I want to know how to add an error with a known standard deviation.
X is the ideall measurements. σ is the standard deviation of the error measurement - known e - error
Xreal = X + e
Xreal = X + σ^2*randn(1) ?
this is not right becouse i will have only positive errors...
please help needed!!

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Wayne King
Wayne King 2012-6-2

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Assume x is your error-free data
x = cos(pi/4*(0:99));
% now add the error, let the standard deviation be 2
x = x+ 2*randn(size(x));
You do not want to multiply randn() by the variance, you multiply by the standard deviation.

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Valeriu
Valeriu 2012-6-2
Thanks a lot for the answer!!
ANetTow
ANetTow 2012-7-17
Is there any difference between using randn as above or using random? For example, using the same items from above:
x = random('norm', x, 2);

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