A transportation researcher is funded to estimate the proportion of automobile tires with an unsafe tread thickness in a small northern state. The researcher randomly selects one month during each of the four seasons for taking the measurements. During each of the four selected months, the researcher randomly selects 500 cars from the list of registered cars in the state and then measures the tread thickness of the four tires on each of the selected cars.
a. What is the population of interest?
b. What is the sampling frame?
c. What biases if any may result from using the data from this study to obtain the estimated proportion of cars with an unsafe thread thickness?