... 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 y¯ These differences from the mean are called deviations. Cases for which yi is less than y¯ produce negative deviations (yiy¯<0) Cases for which yi is greater than y¯ produce positive deviations (yiy¯>0) 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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