Statistics is that branch of mathematics which deals with the collection and interpretation of data. The data may be represented in different graphical forms such as bar graphs, histogram, ogive curve, and pie chart. This representation of data reveals certain salient features of the data. These values of the data is called measure of central tendency. The various measure of central tendencies are mean, median and mode. A measure of central tendency gives us the rough idea of where data points are centered. But in order to make more accurate interpretation of central values of the data we should also have an idea of how the data are scattered around the measure of central tendency. Thus we can say that the measure of central tendency is not sufficient to give the complete information about a given data. There is another factor which is required to be studied under the statistics is the variability of the data. This variability of data is called measure of dispersion. Thus the important measure of dispersion is the mean deviation from mean or median. We have discussed about the various method of calculating mean, median, and mode in class IXth. In this chapter we shall discuss about the various methods of calculating deviation about mean and median of the data. We will also discuss about the variance and standard deviation of the data.