Jump to content

which form of standard deviation


Bryn

Recommended Posts

can someone give me a explernation of when to use the form

 

[math]\sigma^2 = \frac{\sum(x-\mu)^2}{n}[/math]

 

and when to use

 

[math]s^2 = \frac{\sum(x-\bar{x})^2}{n-1}[/math]

Link to comment
Share on other sites

Well, it really doesn't make that much of a difference to which one you use. But anyways, here is the explanation:

 

The first form that you have mentioned needs to be used when you are going to take the standard deviation OF THE WHOLE population.

 

The second one is when you take a sample of a population ie. only some of the population

 

{edit}

 

Got them the wrong way round :)

Link to comment
Share on other sites

No you didn't.

 

The n form is the population standard deviation, the n - 1 form is the sample standard deviation. That's why the top one uses Mu, denoting the population mean, and the bottom one uses XBar, denoting the sample mean.

Link to comment
Share on other sites

oh sorry, if u do maths a levels especially the stats modules then u will understand it. you don need to know this now

 

I had the same problem when doing GCSE cos the course isnt really exact on which formula ur meant to use. Its better to ask you teacher.

Link to comment
Share on other sites

why dont u go to the exam boards website,

if its AQA http://www.aqa.org.uk/qual/gcse.html and select which maths ur doing, and then view the specifications.

 

and for edexcel

 

http://www.edexcel.org.uk/qualifications/QualificationSubject.aspx?id=50009

 

the specs should have everything u need in there. there are should also be some past papers with answers, so if u look at them u should get an idea of which form to use

Link to comment
Share on other sites

You didn't understand, the edit WAS the correction that I had made! I put it the other way round, after the edit, it was correct.

Sorry. Reading your post as it stands, there is no way of knowing that.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.