The National Assessment of Educational Progress ( NAEP) includes a mathematics test for eigth-grade students. Scores on the test range from 0 to 500. Suppose that you give the NAEP test to an SRS of 1100 8th-graders from a large population in which the scores have mean 280 and standard deviation 129 . The mean x¯¯¯ will vary if you take repeated samples.
The sampling distribution of x¯¯¯ is approximately Normal. It has mean 280 .
What is its standard deviation
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