The report “Undergraduate Students and Credit Cards in 2004: An Analysis of Usage Rates and Trends” (Nellie Mae, May 2005) estimated that 21% of undergraduates with credit cards pay them off each month and that the average outstanding balance on undergraduates’ credit cards is $2169. These estimates were based on an online survey that was sent to 1260 students.

Responses were received from 132 of these students. Is it reasonable to generalize the reported estimates to the population of all undergraduate students? Address at least two possible sources of bias in your answer.