# Statistics with Probability

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• Chapter 9,4. Problem 13
• #12991

Alphatron
Participant

For that problem, neither the mean nor the standard deviation are given to you, so I calculated them myself. However, I want to make sure of something. Once you find the sqaure root of the variance, is that the standard deviation of X or X bar? Do you still have to divide that number by the square root of the sample size, or can you use it as is (as your std. dev of your X bar).

Also, we didn’t have to include pictures in that assignment, did we? I sure hope not…

#16265

Ezra Halleck
Participant

I think the confusion is that there are 2 things called xbar. There is lowercase xbar which is just the mean of your data. Then there is the uppercase Xbar which is the theoretical distribution of lowercase xbar’s. The standard deviation of the sample S is NOT the standard deviation for Xbar. To find the standard deviation of Xbar you must divide by the square root of the sample size.

As far as pictures, the answer is no, you do not need a picture, although I strongly suggest that you make one and keep it as part of your notes.

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