How to decide cutoff frequency in designing low pass filter??

I am using matlab built in function fir1(N,Wn) for designing low pass filter using window technique. I have no idea is there any particular criteria to follow to decide normalized cutoff frequency?

[Adam]

Depends what your data is and what you are trying to do with it in low pass filtering it.

[Kapal]

load oct1_fast % that loads actual signal with 3661 samples Smeasured=0.0189; % Sigma Measured mu=0; n2 = normrnd(mu,Smeasured,[length(x) 1]);

now I want to filter n2 (basically a white noise) and I am using window technique but how to get cutoff frequency? I know normalized cutoff (2*cutoff/fs) will be used in fir1 function .