The Standard Deviation: The standard deviation is the most frequently used measure of deviation. In simple terms, it is defined as "Root - Means - Square - Deviation." It is denoted by the Greek letter sigma c; or by the initials S.D.> The Mean: The arithmetic mean is widely used in statistical calculation. It is sometimes simply called Mean. To obtain the mean, the individual observations are first added together, and then divided by the number of observations.">