These three concepts are the basics of the following topics, therefore it is very important to understand them.[br][br][list][list] [b][*] Mean[/b]: The mean usually refers either to the expected value of a random variable or to the arithmetic average of a set of data [b][*]Variance:[/b] A measure of variability defined as the expected value of the square of the random variable around its mean[b][*] Standard Deviation:[/b] The positive square root of the variance. The standard deviation is the most widely used measure of variability.[/list][/list]