Joe's Research Services was conducting a survey of men. Using a listing of names from the telephone book, Joe's Research Services randomly called 1000 men and asked if they would be interested in participating in a research study. Only 127 men participated in the research study. Of the 873 telephone numbers dialed, 425 men refused, 263 men were not at home, and 185 of the telephone numbers were no longer in service. What error(s) might have occurred in this study