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

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.

[Shawn]

This is the code I am using:

 if true
 % code
load a.MAT                  % load data file
a=A16;                      % define data set
at=a(:,1);                  % assign time data to variable
a1=a(:,2);                  % assign amplitude data to variable
a1s=a1-mean(a1);            % DC offset shift to zero
Q = a1s((at>=0.2 & at<=.6)); % select noise range
W = at((at>=0.2 & at<=.6)); % select time data for noise range
  end

However the selection of the noise range executes vary slowly. Does anyone have a suggestion for stream lining the code so that MATLAB can execute the data selection more quickly.

[Rick Rosson]

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]

Total rows of entire data set = 189857

Time steps are uniformly spaced at 9.06 micro seconds

Answer 1

[Rick Rosson]

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

HTH.