# Frequency Table Standard Deviation

• Dec 11th 2009, 03:16 PM
hellotarina
Frequency Table Standard Deviation
Okay I need help. I'm studying for a test, and I keep trying to do this problem over and overrr.

This is the table.

Classes ---- Frequency
10-13 ---- 1
14-17 ---- 0
18-21 ---- 15
22-25 ---- 7
26-29 ---- 2

I keep getting 7.93 but my answer sheet says it's supposed to be 3.2! (Headbang)

What am I doing wrong?
I would post my work but I don't know how to (Lipssealed).

• Dec 12th 2009, 01:49 AM
mr fantastic
Quote:

Originally Posted by hellotarina
Okay I need help. I'm studying for a test, and I keep trying to do this problem over and overrr.

This is the table.

Classes ---- Frequency
10-13 ---- 1
14-17 ---- 0
18-21 ---- 15
22-25 ---- 7
26-29 ---- 2

I keep getting 7.93 but my answer sheet says it's supposed to be 3.2! (Headbang)

What am I doing wrong?
I would post my work but I don't know how to (Lipssealed).

It would help if you said what the actual question was.
• Dec 12th 2009, 03:36 AM
hellotarina
The question just tells me to find The Standard Deviation of the frequency table posted above. It is to compare standard deviation done by raw data (3.2) and standard deviation done by a grouped data. I know the answer is meant to be 3.2 as well but I don't know how they get this.
• Dec 12th 2009, 03:58 AM
mr fantastic
Quote:

Originally Posted by hellotarina
The question just tells me to find The Standard Deviation of the frequency table posted above. It is to compare standard deviation done by raw data (3.2) and standard deviation done by a grouped data. I know the answer is meant to be 3.2 as well but I don't know how they get this.

I get 3.24.
• Dec 12th 2009, 04:26 AM
hellotarina
√n[Sigma(f * x^2)] - [Sigma(f*x]^2
_____________________________ = s
n(n-1)

I get

√ [29(11213.9)] - (523.5)^2
______________________
29(28)

So,

325203.1 - 274052.25
___________________
812

51151
_____ = √62.99
812

And then I get 7.93 :(

I got all the results from:

Classes----Frequency ---- Midpoint (x) ---- x^2
10-13 ---- 1 -----11.5------132.25
14-17 ---- 0 -----15.5 ----- 240.25
18-21 ---- 15-----19.5-----380.25
22-25 ---- 7------23.5-----552.25
26-29 ---- 2------27.5-----756.25

And then of course:

x^2*f
132.25
0
5703.75
3865.4
1512.5
Sigma=11213.9

The 523.5 I got from Sigmaf*x
• Dec 12th 2009, 04:43 AM
mr fantastic
Quote:

Originally Posted by hellotarina
√n[Sigma(f * x^2)] - [Sigma(f*x]^2
_____________________________ = s
n(n-1)

I get

√ [29(11213.9)] - (523.5)^2
______________________
29(28)

So,

325203.1 - 274052.25
___________________
812

51151
_____ = √62.99
812

And then I get 7.93 :(

I got all the results from:

Classes----Frequency ---- Midpoint (x) ---- x^2
10-13 ---- 1 -----11.5------132.25
14-17 ---- 0 -----15.5 ----- 240.25
18-21 ---- 15-----19.5-----380.25
22-25 ---- 7------23.5-----552.25
26-29 ---- 2------27.5-----756.25

And then of course:

x^2*f
132.25
0
5703.75
3865.4
1512.5
Sigma=11213.9

The 523.5 I got from Sigmaf*x

There are several mistakes (although some might just be typos). But the fact that your value for n is wrong is a very bad mistake, one that you should have realised yourself.
• Dec 12th 2009, 04:50 AM
hellotarina
Quote:

Originally Posted by mr fantastic
There are several mistakes (although some might just be typos). But the fact that your value for n is wrong is a very bad mistake, one that you should have realised yourself.

I'm a noob at this. It's my first stats class. And I just realised what you mean. I confused the classes with the n. However, I get 600 when I do n(n-1) and that only makes the standard deviation higher...

Farther from the answer I'm supposed to be getting.

51151
_____= 85.3
600

√85.3 = 9.24
• Dec 12th 2009, 04:55 AM
hellotarina
Holy...

I just got the answer!!! Hehe. (Rock)(Rock)

3.23 (Evilgrin)

You're a genius, sir!
• Dec 12th 2009, 04:59 AM
mr fantastic
Quote:

Originally Posted by hellotarina
I'm a noob at this. It's my first stats class. And I just realised what you mean. I confused the classes with the n. However, I get 600 when I do n(n-1) and that only makes the standard deviation higher...

Farther from the answer I'm supposed to be getting.

51151
_____= 85.3
600

√85.3 = 9.24

I have given you a link with the formula and which shows how to set things out. It's now nothing more than arithmetic.

Following the link - carefully - I got $s^2 = \frac{1}{24} \left( 11214.25 - \frac{523.5^2}{25}\right) = 10.5067 \Rightarrow s = 3.24$.

I cannot push the buttons on the calculator for you. And I should not have to be telling you the value of n. It's not a matter of being a 'noob'. It's simply a matter of doing things carefully.
• Dec 12th 2009, 05:00 AM
mr fantastic
Quote:

Originally Posted by hellotarina
Holy...

I just got the answer!!! Hehe. (Rock)(Rock)

3.23 (Evilgrin)

You're a genius, sir!

See ....! I told you so. Simply arithmetic. A matter of taking care. Being a noob does not matter.

(And yes, I am, aren't I!)