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### Standard Deviation

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Date: 05/17/99 at 15:53:03
From: Rob
Subject: Statistics, Standard Deviation

What is standard deviation?
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```
Date: 05/17/99 at 18:03:21
From: Doctor Pat
Subject: Re: Statistics, Standard Deviation

Rob,

The general idea of a standard deviation is that it is a typical
amount to be different. Think of it this way: suppose the average
height of 16-year-old males is 70 inches - but almost no one is
EXACTLY 70 inches tall. If someone is 71 inches high you will probably
still say he is pretty normal in height. Maybe even 72 inches, but at
73 or 74 inches you would start to say that that person is NOT
typical; he is on the tall side.

This is sort of what the standard deviation does: it gives us a number
to put on sets that says this is (more or less) an unusual measure for
this population.

To find it follow these easy steps:

1) Find the average of the set

2) Create a new set (the deviations) by subtracting the average from
each of the original numbers.

3) Square each number in the set of deviations to make a set of
squared deviations

4) Average the set of squared deviations

5) Take the square root so that the answer has the same units as the
original measures  (ft or inches or whatever)

This is called the root mean square deviation  because it is the ROOT
of the MEAN of the SQUARE of the DEVIATIONS.

Hope that helps.

- Doctor Pat, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Statistics

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