Unit 6 Practice Free Response Question
SCORING:
Both parts essentially correct
How much do Americans pay for coffee on average? In a 2015 survey of 105 randomly selected U.S.
adults, the respondents were asked what price they paid for their most recent purchase of coffee.
The sample data had a mean of $3.42 and a standard deviation of $0.75.
(a) Construct and interpret a 95% confidence interval for the mean price paid for coffee in 2015 by all
U.S. adults.
3.42 +- 1.984(0.75/sqrt105) = (3.27, 3.57) for df = 100
We are 95% confident that the interval (3.27, 3.57) captures the true mean price paid for coffee in
2015 by all US adults
(b) In 2013, tax receipts show that Americans paid an average of $2.98 per coffee. Do the 2015
survey results provide convincing evidence that the average price Americans pay for coffee has
changed? Explain your reasoning.
Yes, because $2.98 is lower than even the lowest part of the confidence interval.