7.2

1. Online courses: A sample of 255 students who were taking online courses were asked to describe their overall impression of online learning on a scale of 1-7, with 7 representing the most favorable impression. The average score was 5.21, and the standard deviation was 0.84. (a) Construct a 99.9% confidence interval for the mean score. (b) Assume that the mean score for students taking traditional courses is 5.47. A college that offers online courses claims that the mean scores for online courses and traditional courses are the same. Does the confidence interval contradict this claim? Explain.

2. Get an education: In 2012 the General Social Survey asked 848 adults how many years of education they had. The sample mean was 8.47 years with a standard deviation of 8.99 years.

(a) Construct an 80% confidence interval for the mean number of years of education. Round the answers to at least two decimal places.

3. Let's go to the movies: A random sample of 42 Hollywood movies made in the last 10 years had a mean length of 129.2 minutes, with a standard deviation of 13.9 minutes.

(a) Construct a 99% confidence interval for the true mean length of all Hollywood movies in the last 10 years. Round the answers to at least one decimal place.

7.3

4. Smart phone: Among 245 cell phone owners aged 18-24 surveyed, 106 said their phone was an Android phone.

(a) Find a point estimate for the proportion of cell phone owners aged 18-24 who have an Android phone. Round the answer to at least three decimal places.

5. Working at home: According to the U.S. Census Bureau, 30%of men who worked at home were college graduates. In a sample of 487 women who worked at home, 169 were college graduates.

(a) Find a point estimate for the proportion of college graduates among women who work at home. Round the answer to at least three decimal places.

6. Internet service: An Internet service provider sampled 538 customers and found that 78 of them experienced an interruption in high-speed service during the previous month.

(a) Find a point estimate for the population of all customers who experienced an interruption. Round the answer to at least three decimal places.

7. SAT scores: A college admissions officer sampled 126 entering freshmen and found that 45 of them scored more than 570 on the math SAT.

(a) Find a point estimate for the proportion of all entering freshmen at this college who scored more than 570 on the math SAT. Round the answer to at least three decimal places.

8. Contaminated water: In a sample of 42 water specimens taken from a construction site, 26

contained detectable levels of lead.

(a) Construct a 90% confidence interval for the proportion of water specimens that contain detectable levels of lead. Round the answer to at least three decimal places.