# 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
$\displaystyle \sigma = \sum_1^n \sqrt{\frac{(x_i - \bar{x})^2}{n}}$

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

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

No the sample SD:

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

Or for the estimator of the population SD

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

or sample SD for grouped data:

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

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

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.