# How do mean and standard deviation change after discarding outliers? [closed]

**Sara measured the time in minutes between cars passing her camp near a desert road over a two hour period

The times she measured were 6, 6, 8, 9, 10, 11, 13, 13, 20, 24.

She calculated the mean and standard deviation for this set of data. (She decided to reject the two outliers (20 and 24)

How do the new mean and standard deviation compare to her original ones?

A The mean and standard deviation both decrease.

B The mean and standard deviation both increase.

C The mean stays the same and the standard deviation decreases.

D The mean decreases and the standard deviation increases.

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## closed as too localized by MJD, Alexander Gruber♦, Micah, Davide Giraudo, tomaszMar 13 '13 at 18:54

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And your question is? – Valtteri Mar 13 '13 at 17:22
hi valtteri thanks for your comment my question is are my answer A is correct < thanks valtteri have a nice day ♥ – Momen Osama Mar 13 '13 at 17:24
Why did you answer A? – Valtteri Mar 13 '13 at 17:26
Because if deletes values it ​​will decrease and will decrease total and that lead to decrease the mean, also decrease the dispersion of the values ​​so will decrease the standard deviation > – Momen Osama Mar 13 '13 at 17:29
Do you have any reason to think the way you think is wrong? – Valtteri Mar 13 '13 at 17:31

Yes, you are correct: and for the reasons you give in your comments, too, [sort of for the mean (the divisor = sample size, will also decrease), but overall the mean will decrease]. As you state, as well, with respect to the standard deviation, the dispersion will certainly decrease without the values $20$ and $24$ removed. And the standard deviation will decrease as a result (the remaining values are more closely "lumped" together).