# Thread: finding the standard deviation and mean from frequency table

1. ## finding the standard deviation and mean from frequency table

I have a frequency table showing age distribition in Auckland supercity election 2010 and I have in the coloumns I have Age, frequency, percentage, upperbondary, class midpoint, class width, histogram height.
In my age rows I have things such as 20 to under 40, 40 to under 50, 50 to under 60, 60 to under 70, and 70 to under 80.
Now my question is how do I find the mean age and standard deviation age from that frequency table.
I have casio fx-82MS calculator and I switched to sd mode and cleared the memory and I enter the data from upper boundary and I press the M+ button after i enter each of these data's and once I finish I press shift 2(s-var) then select the mean and stadard deviation. But the answer I get is wrong. Can anyone tell me what I'm doing wrong????

2. ## Re: finding the standard deviation and mean from frequency table

You need to use the class midpoints - not the (class) upper boundary values.

3. ## Re: finding the standard deviation and mean from frequency table

I tried entering it from class midpoint but I still get wrong answer. I get 54 for mean and 15.2 for standard deviation. Here is my raw data for class midpoint: 30, 45, 55, 65, 75. The answer is 58.63 years for mean and 14.89 years for standard deviation. But my question here is, how do we get them?

4. ## Re: finding the standard deviation and mean from frequency table

You should start with the mean. You have given the 5 midpoints. Now, what are the 5 class frequencies?

5. ## Re: finding the standard deviation and mean from frequency table

How do you 'know' what the answer is? Is it reported along with the table or something? You most likely won't be able to get the exact same answer because when the data gets binned you lose quite a bit of information.

Quick example:
Real data: -0.9, .1
mean of actual data = -.4
Which corresponds to the following binned Data:
(-1, 0]: 1
(0, 1] : 1
Mean calculated using binned data: 0

These don't match up but we shouldn't expect that they could possibly perfectly match up.

6. ## Re: finding the standard deviation and mean from frequency table

The frequencies are in order: 30, 20, 48, 56, 58

7. ## Re: finding the standard deviation and mean from frequency table

Okay, so then it is straight-forward to compute the mean:

(30)*(30) + (20)*(45) + (48)*(55) + (56)*(65) + (58)*(75) = 12430

Thus, the mean is:

12430/212 = 58.6321

Do you follow?

8. ## Re: finding the standard deviation and mean from frequency table

thanks it worked but the problem is how do i enter them to calculator? thats the problem, when finding standard deviation.

9. ## Re: finding the standard deviation and mean from frequency table

Originally Posted by newbiestats
thanks it worked but the problem is how do i enter them to calculator? thats the problem, when finding standard deviation.
Well, no, that's not the problem, newbiestats. In short, it is obvious that you didn't know how to correctly compute the mean to begin with---set calculator issues aside. Mkay.

10. ## Re: finding the standard deviation and mean from frequency table

In my calculator I have to go on sd mode and enter those data's then press M+ and then compute the mean and standard deviation but it doesnt work. I tried putting numbers like this 30;30 and press M+, 20;45 and press M+ and such and to compute the standard deviation and mean age but still didnt work.

12. ## Re: finding the standard deviation and mean from frequency table

Originally Posted by BGM

I did follow it and yet I stil get wrong answer. I still get 54 as mean from using those calculator instructions and I also get 17.46 as standard deviation which are both wrong according to the paper. The correct answer for mean is 58.63 and standard deviation is 14.89

13. ## Re: finding the standard deviation and mean from frequency table

And here is the picture of frequency table with answer below the tables. I really need help on this.

14. ## Re: finding the standard deviation and mean from frequency table

Which part of the following was giving you trouble?
Originally Posted by Dragan
Okay, so then it is straight-forward to compute the mean:

(30)*(30) + (20)*(45) + (48)*(55) + (56)*(65) + (58)*(75) = 12430

Thus, the mean is:

12430/212 = 58.6321

Do you follow?

15. ## Re: finding the standard deviation and mean from frequency table

Originally Posted by Dason
Which part of the following was giving you trouble?
ok i understand that part, but how do I find the standard deviation?