... 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  These differences from the mean are called deviations. Cases for which yi is less than  produce negative deviations  Cases for which yi is greater than  produce positive deviations  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.

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