The mean is the measure of center most affected by outliers.
Let's say you have the numbers: 1, 2, 2, 3, 4, 5, 6, 7, 8, 100.
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The mean (average) is (1+2+2+3+4+5+6+7+8+100) / 10 = 13.8. The big number 100 makes the mean really high.
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The median (middle number) is 4.5 because when you put the numbers in order, the two middle ones are 4 and 5.
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The mode (most common number) is 2, because it shows up the most.
So, the mean changed a lot because of that big 100, but the median and mode stayed the same! That's why the mean is most affected by outliers.