According to the website, the mean highway fuel efficiency of all 1078 car models tested in 1990 was 22.3 miles per gallon (mpg) and the standard deviation was 5.7 mpg. In 2015, when 1279 car models were tested, the mean fuel efficiency was 28.9 mpg and the standard deviation was 9.9 mpg. The distribution of fuel efficiency in 1990 is approximately normal, but the 2015 distribution has several high outliers. Suppose we take separate random samples of 35 car models from 1990 and 40 from 2015 and calculate the difference in sample mean fuel efficiency in 1990 and 2015. Find the mean of the sampling distribution.
a) Calculate the mean of the sampling distribution.
b) Interpret the meaning of the sampling distribution mean.