Cause analysis: a, the median first arranges all the numbers from small to large, that is, 1, 2, 2, 3, 4, 4, 5, so the median is 3; After removing 3, the order is 1, 2, 2, 4, 4, 5, the median is 2, and the average of 4 is still equal to 3.

B, the average sum of all numbers divided by the number, 2 1 divided by 7 equals 3; After removing 3, 18 divided by 6 equals 3, so it is the same.

C, the mode can't reach 3, and the most common numbers are 2 and 4, so the mode hasn't changed, they are both 2 and 4.

D, variance, before removing 3, it can be calculated that the variance is 12 divided by 7 equals to 712; After removing 3, it can be calculated that 12 divided by 6 equals 2 according to the formula, so the variance before and after has changed.

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