From the Chicago Dispatcher
June 2009

By George Lutfallah

Time of Surveys

Your study indicates that “the majority of interviews occurred between the hours of 12:00 p.m. and 3:30 p.m. and took approximately 20 minutes to complete.”

This would mean that virtually no drivers who operate on the night shift exclusively were even included in the study at all. My perception is that night drivers are more likely to be risk takers than day drivers who are more likely to be family men and women. It can be reasonably argued that risk takers will show higher income returns. This would suggest more selection bias in your study.