单选题 A sample of size n=25 is selected from a normal population. This sample has a mean of 15 and a sample variance of 4. What is the standard error of the sample mean?

A. 2.0.
B. 0.4.
C. 0.8.

【正确答案】 B

【答案解析】The standard error of the sample mean is estimated by dividing the standard deviation of the sample by the square root of the sample size. The standard deviation of the sample is calculated by taking the positive square root of the sample variance [*]. Applying the formula: [*]=s/ (n)^1/2=2/(25 )^1/2=2/5=0.4.