Question

When the outliers are removed, how does the mean change?

Answer 1

Usually it will decrease the value of the mean since you are removing values from the data set.

Answer 2

It could also increase the mean though. It depends what the values are. For example:

Data set 1:
1, 2, 3, 4, 5, 20. -> mean is 5.833
remove outlier:
1, 2, 3, 4, 5 -> mean is 5 (it decreased)

Data set 2:
1, 100, 200, 300, 400, 500 -> mean is 250.17
remove outlier:
100, 200, 300, 400, 500 -> mean is 300 (it increased)