# Standard Deviation problem/help

• Jan 21st 2010, 06:05 PM
paramed
Standard Deviation problem/help
Hi all,

I need to work out the standard dev for the following table

Crushing Strength N/mm2
25
28
31
34
37
40
Frequency (f)
15
30
42
36
24
18

However I have used a spreadsheet to calculate the answer and I'm coming up with 10.46422477. This is also the figure I get when working it out on the calculator.

The problem is the mean I worked manually using formula: (sigma) crushing strength x frequency / (sigma) frequency, so 5349/165 comes to 32.41818182. But the mean using excel comes to 27.5.

Can someone work out the standard deviation for me so I can see where I'm going wrong?

Thanks ppl
• Jan 22nd 2010, 04:14 AM
CaptainBlack
Quote:

Originally Posted by paramed
Hi all,

I need to work out the standard dev for the following table

Crushing Strength N/mm2
25
28
31
34
37
40
Frequency (f)
15
30
42
36
24
18

However I have used a spreadsheet to calculate the answer and I'm coming up with 10.46422477. This is also the figure I get when working it out on the calculator.

The problem is the mean I worked manually using formula: (sigma) crushing strength x frequency / (sigma) frequency, so 5349/165 comes to 32.41818182. But the mean using excel comes to 27.5.

Can someone work out the standard deviation for me so I can see where I'm going wrong?

Thanks ppl

Check the Excel calculation for the mean. Also check your calculation of the SD, it is implausible that all the data is within less than 1 SD of the mean.

CB