Click here for latest version. A look at a graph of the sampling distribution of M d makes the problem more concrete. The mean of the sampling distribution is 10 and the standard deviation is 2.35. The graph shows the mean of the distribution.
Tick marks are one standard deviation apart.
The probability can be determined by computing the number of standard deviations above the mean 15 is. It follows that the probability of a difference between the drug and no-drug means of 15 or larger is .017.