Perhaps the most important measure of location is the mean, or average value, for a variable. The mean provides a measure of central location for the data. If the data are for a sample, the mean is denoted by ( x bar); if the data are for a population, the mean is denoted by the Greek letter μ.
In statistical formulas, it is customary to denote the value of variable x for the first observation by X1, the value of variable x for the second observation by X2, and so on. In general, the value of variable x for the ith observation is denoted by Xi. For a sample with n observations, the formula for the sample mean is as follows.
