Inference and Experiments Practice Test

Researchers want to study whether people who sleep fewer hours tend to have higher stress. They recruit 300 volunteers from a large company by emailing all employees; those who respond complete a survey reporting average sleep hours and a stress score. Which conclusion is justified based on the design?

(Assume accurate self-reporting.)

A state transportation agency wanted to estimate the average time (in minutes) that commuters spend waiting for a train during weekday mornings. The agency randomly selected 15 train stations from all stations in the state and then, at each selected station, recorded the waiting time for the first 20 commuters who arrived between 7:00 and 9:00 AM on a single Tuesday. Which conclusion is justified based on the design of this study?