Statistics question?

Question

I just want to make sure I got the right answer (mean=6; standard devation=0). I suck at statistics so I might be way off.

Determine the mean and the standard deviation of the following frequency distribution.

Class
0 up to 5
5 up to 10
10 up to 15
15 up to 20
20 up to 25


Frequency
2
7
12
6
3

Answer 1

To find the mean,
multiply the midpoint of each class by the frequency.
Do this for all 5 classes. Total the 5 products and divide by n.
2.5 * 2 = 5
7.5 * 7 = 52.5
12.5 * 12 = 150
17.5 * 6 = 105
22.5 * 3 = 67.5
Total = 380
n = 2 + 7 + 12 + 6 + 3 = 30
Mean = 380 / 30 = 12.6666

To find the standard deviation:
1) Subtract the mean from the class midpoint. That is, find (M - xbar). For the first class (2.5 - 12.6666 = -10.1666), for the second class (7.5 - 12.6666 = -5.1666), etc.

2) Square the difference between the class midpoint and the mean. For the first class it would be (-10.1666^2 = 103.36), for the second class (-5.1666^2 = 26.69), etc.

3) Multiply the squared difference between the class midpoint and the mean by the class frequency. For the first class the value is 2*(2.5 - 12.6666)^2 = 206.72. For the second class the value is 7*(7.5 - 12.6666)^2 = 186.86, and so on.

4) Add all the results from Step 3. The total is 824.1667.
s = sqrt [824.1667 / (30 - 1)] = 5.331

I've uploaded an Excel spreadsheet of the problem at:
http://i12.tinypic.com/4tr0j9c.jpg

or you can pay $3.34 for the solution at:
http://www.brainmass.com/homework-help/statistics/all-topics/93131

Good luck in your studies,
~ Mitch ~

Answer 2

Here is a site that will do it for you:

http://www.ltcconline.net/greenl/java/Statistics/FrequencyData/FrequencyData.html