单项选择题 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（）

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（）