Statistics for Business: Decision Making and Analysis, 3rd Edition

... On average, each song needs a bit less than 3.8 MB. Notice that the average size of a song is larger than the median size 3.5 MB.

Averaging also produces a summary statistic that measures the amount of variation in a numerical variable. Consider how far each observation is from $\overline{y}$ These differences from the mean are called deviations. Cases for which y_i is less than $\overline{y}$ produce negative deviations $\left(y_i-\overline{y}<0\right)$ Cases for which y_i is greater than $\overline{y}$ produce positive deviations $\left(y_i-\overline{y}>0\right)$ The average squared deviation from the mean is called the variance. We use the symbol s² for the variance, with the exponent 2 to remind you that we square the deviations before averaging them.

variance

The average of the squared deviations from the mean, denoted by the ...