Improve company productivity with a Business Account.Sign Up

x
  • Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 1210
  • Last Modified:

Formula for computing standard deviation

Does anyone out there have a formula for computing Standard deviation for a set of given numbers.

P.S. This formula can not use any arrays, and also the numbers  will becoming from a file.
0
delbrad
Asked:
delbrad
1 Solution
 
interCommented:
Lets assume that we have a file which is a text file and each line contains one data (you can easily modify this if you want):

The formula is:

  mean = SumOf(x1, x2, ..., xN) / N;
  variance = SumOf((x[i]-mean)^2/(N^2));
  stddev = sqrt(variance);

function StdDev(FileName : string):real;
var
  mean : real;
  nofitems : integer;
  F : text;
  S : string;
  variance : real;
begin
  AssignFile(F, FileName); Reset(F);
  // first compute the mean of the table
  nofitems := 0; mean := 0;
  while not eof(F) do
  begin
    readln(F, S);
    mean := mean + StrToFloat(S);
    Inc(nofitems);
  end;
  // divide total by number of items
  mean  := mean / nofitems;
  CloseFile(F);
  AssignFile(F, FileName); Reset(F);
  // then compute the variance
  variance := 0;
  while not eof(F) do
  begin
    readln(F, S);
    variance := variance + sqr(StrToFloat(S) - mean);
    Inc(nofitems);
  end;
  // normalize it
  variance := variance / sqr(nofitems);
  CloseFile(F);
  result := sqrt(Result); // std is the square root of variance
end;


See you around friend!  
Igor
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

Featured Post

Upgrade your Question Security!

Your question, your audience. Choose who sees your identity—and your question—with question security.

Tackle projects and never again get stuck behind a technical roadblock.
Join Now