# Thread: whats the equation for standard deviation?

1. ## whats the equation for standard deviation?

as stated in post heading. *equation meant to be formula lol

Im confused, could somebody give me the forumla for standard deviation? x

2. Originally Posted by leaaa
as stated in post heading. *equation meant to be formula lol

Im confused, could somebody give me the forumla for standard deviation? x
$\sigma = \sum_1^n \sqrt{\frac{(x_i - \bar{x})^2}{n}}$

3. Originally Posted by colby2152
$\sigma = \sum_1^n \sqrt{\frac{(x_i - \bar{x})^2}{n}}$
Keep in mind that is for a population set. For a sample, you must divide by n-1!

4. Originally Posted by colby2152
$\sigma = \sum_1^n \sqrt{\frac{(x_i - \bar{x})^2}{n}}$

No the sample SD:

$
s = \sqrt{\frac{1}{n}\sum_1^n (x_i - \bar{x})^2}
$

Or for the estimator of the population SD

$
s = \sqrt{\frac{1}{n-1}\sum_1^n (x_i - \bar{x})^2}
$

or sample SD for grouped data:

$
s = \sqrt{ \frac{\sum_1^n n_i (x_i - \bar{x})^2}{\sum_1^n n_i}}
$

where there are $n$ groups with group marks $x_i$ and frequency count $n_i$.

ZB

5. Originally Posted by colby2152
Keep in mind that is for a population set. For a sample, you must divide by n-1!
Where in this context ! is of course an exclamation, NOT a factorial.