[問題] variance=0，sample mean=sample median?
這是1997年AP Statistics測驗裡的一個問題：
A random sample of size 10 was taken from a population.
The sample has a variance of zero.
Which of the following statements must be true?
1.The population also has a variance of zero.
2.The sample mean is equal to the sample median.
3.The ten data points in the sample are equal in numerical value.
(a) 1 only
(b) 2 only
(c) 3 only
(d) 1 and 2
(e) 2 and 3
答案是：(c)
但是不理解，為何答案不是(e)？
∵The sample has a variance of zero.
∴The ten data points in the sample are equal in numerical value.
那麼不是同時也可得出，The sample mean is equal to the sample median.
請問為何上面這句不正確？
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