mystery wrote:

Why do we use these equation?

20 +3*2.5=12.5

20 -3*2.5=27.5

I don't know the reasons.

Thanks!!

oh.. i am sorry...

We use statistical measures to draw conclusion about a sample on which data has been collected..

For example, you may collect data on the height of the students in 5th grade. If you want to report it statistically, the most common way is to report the mean and the standard deviation...

Mean (Average) is the average of the heights in our sample...

to give more information, we provide the standard deviation..

The standard deviation[

SD] tells you how much variation is there is the data..For example , we may find that the S.D for the data is

0.4 feet. The

0.4 feet is the deviation about the

mean
let the mean be 5 feet.

now , if we are asked...what is the value of heights that are 2 standard deviations

from the mean ?

we know SD = 0.4

so 2 SD = 2* 0.4 = 0.8

Now deviation is both to the left and the right of the mean

To the left, we subtract 0.8 from 5 and get one value...which is

4.2
and To the right, we add 0.8 to 5 and get

5.8
These values of

4.2 and

5.8 represent the values that lie TWO SD's from the mean....

Please reply back if you didnt understand...

Hope my explanation was useful..

i tried to draw a diagram ,but it didnt come out right in the post..so i deleted it avoid confusion.

Thanks

Praetorian