Refer to the Journal of Performance of Constructed Facilities (Feb. 2005) study of inspection ratings of all major Denver bridges, Exercise 4.30. Recall that the National Bridge Inspection Standard (NBIS) rating scale ranges from 0 (poorest rating) to 9 (highest rating). Engineers forecast that 9% of all major Denver bridges will have ratings of 4 or below in the year 2020.
a. Use the forecast to approximate the probability that in a random sample of 70 major Denver bridges, at least half will have an inspection rating of 4 or below in 2020.
b. Suppose that you actually observe at least 35 of the sample of 70 bridges with inspection ratings of 4 or below in 2020. What inference can you make? Why?

Data from Exercise 4.30

According to the National Bridge Inspection Standard (NBIS), public bridges over 20 feet in length must be inspected and rated every 2 years. The NBIS rating scale ranges from 0 (poorest rating) to 9 (highest rating). University of Colorado engineers used a probabilistic model to forecast the inspection ratings of all major bridges in Denver. (Journal of Performance of Constructed Facilities, Feb. 2005.) For the year 2020, the engineers forecast that 9% of all major Denver bridges will have ratings of 4 or below.