The average annual expense for groceries in a 2012 random sample of 600 US households is $8562. If the standard deviation of grocery expenses in the population of US households is $1230, compute the
a. Standard error of the sampling distribution of the sample mean that could be used here to estimate the population mean.
b. Margin of error in a 90% confidence interval estimate of the mean annual grocery expense for all American households.
c. Suppose sample size was 1200 rather than 600. Compute the margin of error and the standard error for a 90% confidence interval.