一个统计学的问题Computers are ofter used to randomly generate digits

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一个统计学的问题
Computers are ofter used to randomly generate digits of telephone numbers to be called for surveys.Each digit has the same chance of being selected.Construct a table representing the probability distribution for the digits selected,find its mean,find its standard deviation,and describe the shape of the shape of the probability histogram.

Suppose there are N digits selected:d1,d2,...,dN.
Due to each digit has the same chance to be selected,the probability distribution of the digits selected is a kind of uniform distribution.
So,the mean = (d1 + d2 + ...+ dN)/N.
The mean of square = [(d1)^2 + (d2)^2 + ...+ (dN)^2]/N.
The deviation = the mean of square - the square of mean
= [(d1)^2 + (d2)^2 + ...+ (dN)^2]/N - [(d1 + d2 + ...+ dN)/N]^2
The stardard deviation = the square root of the deviation
= [the deviation]^(1/2)
In the probability histogram,all the digits have the same probability(=1/N),means that all the probability of digits have the the same height(= 1/N).

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