Solved

Formula for computing standard deviation

Posted on 1998-02-14
1
1,198 Views
Last Modified: 2008-03-17
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
Comment
Question by:delbrad
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
1 Comment
 
LVL 5

Accepted Solution

by:
inter earned 50 total points
ID: 1359196
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

Featured Post

Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

A lot of questions regard threads in Delphi.   One of the more specific questions is how to show progress of the thread.   Updating a progressbar from inside a thread is a mistake. A solution to this would be to send a synchronized message to the…
Introduction I have seen many questions in this Delphi topic area where queries in threads are needed or suggested. I know bumped into a similar need. This article will address some of the concepts when dealing with a multithreaded delphi database…
The Email Laundry PDF encryption service allows companies to send confidential encrypted  emails to anybody. The PDF document can also contain attachments that are embedded in the encrypted PDF. The password is randomly generated by The Email Laundr…

734 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question