Standard Deviation formula linear form

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Igor Bogdanov
Igor Bogdanov 2020-8-16
评论： Walter Roberson 2020-8-16
Hello! Have an array X=1xN. How to write the standard deviation formula in matlab (not to use the ‘std’ function)? I know the formula itself, but how is it written in a "linear" form? Thanks! My results is this:
sqrt = ((sum(sum(X)-mean(X)).^2)/(numel(X)-1))
Please, help to find mistake

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回答（1 个）

Walter Roberson
Walter Roberson 2020-8-16
You are assigning the result to a variable named sqrt instead of taking the square root of something.
You should not be taking sum(X) and subtracting the mean from that: you should be taking the sum of (squares of (X minus mean))
  2 个评论
Igor Bogdanov
Igor Bogdanov 2020-8-16
Let me see if I understand this.
standdev = sqrt((sum((X)-mean(X)).^2)/(numel(X)-1))
Walter Roberson
Walter Roberson 2020-8-16
  1. calculate X - mean(X)
  2. take squares of that vector
  3. sum result
  4. calculate number of elements in X, minus 1
  5. divide the sum by that
  6. take the square root of the result of the division
If you go through your code carefully you will see this is not what you are doing.

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