How do I find the std of a specific range of data?

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Shawn 2012-8-1

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I need to determine the standard deviation of the noise in a seismic signal Time domain. The data is in the form of time vs amplitude. I wish to find the standard deviation of the amplitude between time=0.1 and time=0.6. I need to know how to take the std of a range of the data set as opposed to the whole data set.

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Rick Rosson 2012-8-2

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How many rows does the data set contain?

size(a,1)

Also, are the time steps uniformly spaced, or variably spaced? If uniform, what is the time increment?

at(2) - at(1)

Shawn 2012-8-2

Total rows of entire data set = 189857

Time steps are uniformly spaced at 9.06 micro seconds

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Answer 1

Rick Rosson 2012-8-2

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编辑：Rick Rosson 2012-8-2

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Please try:

   tic;
   data = load('a.mat');        % this form is faster than 
                                % command syntax
   t = data.A16(:,1);           % eliminate unnecessary data copies
   a = data.A16(:,2);
   a = a - mean(a);
   idx = (0.2 < t & t < 0.6);   % this operation is time consuming
                                % compute once, use result twice
   Q = a(idx);
   W = t(idx);
   elTime = toc;                % elapsed time