習(xí)題：
　　Exercise:
　　Which of the following statements about the F-distribution and chi-squared distribution is least accurate? Both distributions:
　　A    Are asymmetrical
　　B    Are bound by zero on the left.
　　C    Are defined by degree of freedom.
　　D    As degrees of freedom decrease, both distributions converge to a normal distribution.
　　解析：
　　Answer: D
　　Explanation: As degrees of freedom increase, both distributions converge to a normal distribution.
　　知識(shí)點(diǎn)：
　　Chi-Squared Distribution and F-distribution
　　Chi-Squared Distribution
　　If we have k independent standard normal variables, Z1, Z2, …, Zk, then the sum of their squares, S, has a chi-squared distribution. The variable k is commonly referred to as the degrees of freedom.
　　Properties：
　　l  Because the chi-squared variable is the sum of squared values, it can only take on nonnegative values and is asymmetrical.
　　l  The mean of the distribution is k, and the variance is 2k.
　　l  As k approaches infinity, the chi-squared distribution converges to the normal distribution.
　　F-distribution
　　If U1 and U2 are two independent chi-squared distributions with k1 and k2 degrees of freedom, respectively, then X:
　　follows an F-distribution with parameters K1 and K2.
　　Because the chi-squared PDF is zero for negative values, the F-distribution’s density function is also zero for negative values.
　　As k1 and k2 approach infinity, the F-distribution converges to a normal distribution.
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