Answer- The term standard deviation was first used in 1894 by Carl Pearson. A useful asset for standard deviation is that unlike variance. it is expressed in the same unit as data. This is the most widely used method of variability. The standard deviation indicates the distance of all the scores around the average. According to Gilford “Standard deviation is the square root of the arithmetic means of class deviation of measurement from their means”. Standard deviation is also known as root-mean, class deviation. The standard deviation of the population is denoted by dev (Greek letter sigma) and for the sample S. Standard deviation is the most stable measure of variability. When the significance of the difference between coefficient of correlation, instrument and other data is calculated. Measures of middle tendency are available as median.