Standard Deviation help!

[-westy-]

Hello,

First off, no, this isn't a homework thread, I've done the work.

There's some conflict on my uni course about finding the standard deviation of a set of numbers.


20,26,28,28,28,30,30,32,34,44

Now, the method i'm using is as follows:

1. Find arithmetic mean of numbers = 30
2. Find variation from mean to beginning of range (10) and end of range (14)
3. Square root both numbers:

10 = 3.1623
14 = 3.7416

4. Find values on table of patrial areas under the standardised normal curve (found here: http://www.palgrave.com/business/taylor/taylor1/lecturers/lectures/handouts/hChap5.doc)

10 = 3.1623 = 0.4992
14 = 3.7416 = 0.4999

5. Add values together = 0.991

Standard Deviation of set = 0.991


Have I got this right, or am I on totally the wrong path? The other person has got a Standard Deviation of 5.8651.


Thanks

 

[Amleto]

afaik:
variance = (sum of squares - n *(square of the mean) ] / n

so, no, I don't think what you did is right - you'd need to use all the numbers in the sample, not just the end points

 

[-westy-]

afaik:
variance = (sum of squares - n *(square of the mean) ] / n

so, no, I don't think what you did is right - you'd need to use all the numbers in the sample, not just the end points

Yep, I've had it explained to me now, I'm way out

 

[Infam0us]

Do you not have SPSS at your Uni?