# Calculate 2 standard deviations

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Chris on 28 Aug 2011
Commented: Thomas To on 20 Jan 2020
I am trying to calculate +/- two standard deviations from the mean of a bunch of values. I am not sure if I am using the standard deviation function properly. Would I just multiply by two to get 2 standard deviations?
dmean = mean(values(:,1)); upperstd = dmean+2*std(values(:,1)); lowerstd = dmean-2*std(values(:,1));

Wayne King on 28 Aug 2011
Hi Chris, for a matrix of data
x = randn(10,4);
mean(x)+2*std(x)
mean(x)-2*std(x)
gives the sample means of the columns plus/minus 2 times the standard deviation. The question is whether you really want the sample standard deviation here, or the standard error of the mean.
Wayne
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Thomas To on 20 Jan 2020
Jay, the standard error is calculated by taking the standard deviation and dividing it by the square root of N-th samples used to calculate the standard deviation.

bym on 28 Aug 2011
that is correct