My calculated value for standard deviation without the std command is about a tenth of a point off

So I am trying to calculate standard deviation without using the 'std' command. The code is executing, but the final values for my standard deviation are about a tenth of a point off of what they should be. A

This is the essential code:

while i=1 < length(Feb)
FtotalVar = FtotalVar + ((Faverage - Feb(i))^2)/length(Feb);
i = i + 1;
end
FSDev = FtotalVar^.5;

Feb is a vector with a given input, 28 doubles long. Faverage is the average of the vector. The value that I am getting for the standard deviation is 11.634 while I checked it against the std function which gave me 11.86. Any ideas?

Answer 1

[Star Strider]

You have to divide by (n-1), not n:

Example:

x = randi(100, 1, 50);
stdfcn = @(x) sqrt(sum((x-mean(x)).^2)/(length(x)-1));
compare = [std(x)  stdfcn(x)];