The standard deviation is the most commonly used measure of variability with measurement data.

The range is subject to extremes.

The variance equals the square of the standard deviation.

Things to Know:

1. The standard deviation is a landmark on the normal curve. It reaches from the mean to the point of inflection of the normal distribution.The point of inflection is the point at which the curve changes direction.
   
   
2. Given the mathematics of the normal curve, there are always 68.26% of the scores between the first standard deviation marks around the mean.
   
   
3. So if someone tells you the mean and standard deviation of a distribution, you can tell, the two scores that encompass a little more than 2/3s of your distribution.