additional mathematics. what is the use of standard devitation and variance?

Question

what is the use of standard deviation and variance means?

Answer 1

First know what they are to better understand their uses.
Variance is just what its name says. It tells you in statistics how much something varies from the mean or average. in other words how far from normal is something or what is the likely hood of something beyond the norm occurring.
In statistics it is defined as the average of the squares of the distance of each value from the mean. So if you observed a series of occurrences and recorded them on a graph you would be able to tell what is normal and what is not by looking at how far from the main cluster these occurrences are. Now the standard deviation (SD) is just the square root of the variance in it gives a better way of evaluating how far from the norm is a particular event or situation.
Keep interested in this stuff because as you go further you will see more real life applications. For now its just the math basics and it may seem a little boring but hang on.

Answer 2

If you have a Data with Mean (X-Bar) and standard deviation (S).

The 66% are in X-Bar-S and X-Bar+S.

The 95% are in X-Bar-2S and X-Bar+2S.

The 99% are in X-Bar-3S and X-Bar+3S.

Probably only with this fact can justufy theuse of standard deviation.