X is a discrete random variable that can have values from 0 to infinity, and occurrences of the events X counts are independent. The standard deviation of X is equal to the square root of the mean. The probability distribution of X is a(n) （）distribution.

单项选择题The empirical rule tells us that approximately of the values cluster within a distance of 1 standard deviation from the mean for a bell-shaped distribution（）

A.68%
B.95%
C.99.7%
D.none of the above

单项选择题The reason that we prefer the sample standard deviation as our measure of typical variation instead of the sample variance is that（）

A.the variance is not as accurate a measurement as the standard deviation
B.the standard deviation has been corrected for bias while the variance has not
C.the standard deviation is in the same units as the data values.
D.the variance is in the same units as the data values.

单项选择题The sum of the deviations may be 0.00.To get around this problem，we use the（）of the deviations

A.square
B.absolute value
C.minimum
D.maximum

单项选择题deviation is the distance between a（）and the（）.（）

A.median；mean
B.data point；mean
C.quartile；median
D.data point；median

单项选择题A theorem that allows us to use thenormal probability distribution to approximate the sampling distribution of sample means and sample proportions whenever the sample size is large is known as the（）

A.approximation theorem
B.normal probability theorem
C.central limit theorem
D.central normality theorem