Standard deviation utilizes the mean of a normal distribution, also known as the "bell curve", as a reference point from which to measure the variability by measuring the distance between each score and the mean.  The standard deviation is small when the scores are closely scattered to the mean and large when the scores are widely scattered around the mean.  Simply stated, the standard deviation measures the avergae distance from the mean for each score.

Computing standard deviation can be easy when you have the variance.  Variance measures variability, the measure of the spread of scores in a distribution.  Variance is the average squared distance from the mean or in other words, standard deviation squared.  Therefore, the standard deviation is the square root of the variance.

Calculating standard deviation can be calculated through the following steps:

   1. Measure the distance, also called the deviation, from the mean for each score.

   2. Square each of the distances and calculate the average of the squared distances, this

       would be the variance.

   3. Take the square root of the variance to obtain the standard deviation.

References

Gravetter, F. J. & Forzano, L. B. (2006). Research methods for the behavioral

              sciences, 2nd edition. Belmont, CA: Thomson Wadsworth.

Kranzler, J. H. (2007). Statistics for the terrified, 4th edition. Upper Saddle River, NJ:

              Pearson Education, Inc.