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Mean:  The sum of the numbers in a set of data divided by the number of pieces of data. This is what we usually call "average." Also called the arithmetic mean.

To find the mean of a class's test scores, we need to add all the test scores from that class and divide by the number of tests. The mean takes every score into account and so is affected by extremely high scores and extremely low scores.

Let's find the means (or the averages) for two classes:

 Class 1: 85 + 90 + 78 + 54 + 88 + 76 + 83 + 99 + 49 + 93 + 76 + 90 + 78 + 55 + 89 + 76 + 65 + 81 + 89 + 91 = 20   (Mean for Class 1) Class 2: 76+ 80+ 59+ 89+ 92+ 68+ 55+ 77+ 98+ 95+ 80+ 90+ 96+ 80+ 78+ 65+ 77+ 92+ 89+ 60 20   (Mean for Class 2)

What does this mean? An "average" student in Class 1 scored a 79.25 and an "average" student in Class 2 scored a 79.8. Based on the means, the two classes performed similarly on the test.

Notice that the averages, 79.25 and 79.8, are not equal to any of the test scores. Averages do not have to equal one of the values that it represents. An average just gives us a "general" picture of the values it represents.