Here is a summary of the Boston Marathon completion times for two years. Year
Mean (minutes)
Standard deviation (minutes)
Number of runners
2012
263
50
21,576
31,984
2014
243
52
A sports journalist takes separate random samples of 35 runners from each year. Then they look at the
difference in the mean completion time sampled from each year 2012 - 2014).
What do we know about the shape of the sampling distribution of D2012 22014, and why?