Math Is Fun Forum

netter

Member

Registered: 2011-02-22

Posts: 1

Standard deviation

Hi,

Below are some of my data i'm trying to get the warning and control limits for but in order to get them I need the standard deviation. In excel there are some functions like STDEV and STDEVP but they use the same method by working out the mean as 49.9993 and my true value is actually 50.0001. Is there any way I can work this out without hazzle as I do have a lot of data I have to analyze. I would appreciate the help.

49.9993   
49.9993   
49.9994   
49.9996   
49.9995   
49.9994   
49.9992   
49.9995   
49.9995   
49.9990   
49.9990   
49.9997   
49.9991   
49.9991   
49.9993   
49.9991   
49.9991   
49.9992   
49.9995   
49.9990

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Standard deviation

Hi netter;

I am not following what you need.

The mean of that data is 49.99929.

SD = .000214966

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,649

Re: Standard deviation

hi netter and bobbym,

Just a thought but could it be that the population mean is known to be 50.0001 and the sample mean is 49.99929  ?

You could then ask questions like "Is the process producing values that could reasonably be from the population or does the process need adjusting ?"

Excel seems to have functions for standard deviation of  population and sample, but I cannot find a function that allows you to specify a known mean and then calculate the sd. of a sample.

You would have to make your own function(s) for this, but Excel could certainly do it.

Bob

Last edited by Bob (2011-02-23 01:57:30)

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob