1. ## Measures of variability?

Find the arithmetic mean and the standard deviation of each frequency distribution.

Class Limit Frequency
1-5 2
5-9 8
9-13 15
13-17 6
17-21 38
21-25 31
25-29 13
29-33 7

I managed to find the arithmetic mean doing 2320/120 giving me 19.33

But the standard deviation is giving me trouble

I know the formula is (x-mean)^2 times F

and I did that for example (3-19.33)^2=531.8 (3 being the class mark)

I did that for all them and added them up to get 5040

Then I did stat dev= square root (5040/100) and I get 7.0009 but checking my answer apparently it is wrong.

Is there any way I can do this with a calculator?

2. ## Re: Measures of variability?

Originally Posted by homeylova223
Find the arithmetic mean and the standard deviation of each frequency distribution.

Class Limit Frequency
1-5 2
5-9 8
9-13 15
13-17 6
17-21 38
21-25 31
25-29 13
29-33 7

I managed to find the arithmetic mean doing 2320/120 giving me 19.33

But the standard deviation is giving me trouble

I know the formula is (x-mean)^2 times F

and I did that for example (3-19.33)^2=531.8 (3 being the class mark)

I did that for all them
You need to make sure you do this for each sample in the class, twice in the first, eight times in the second and so on. Add them together divide them by n-1 and take the square root.