Professor Lilli Gans wishes to study the relationship between a person's zodiac sign and his or her political opinions. She obtains the birthdays of all 816 students enrolled in her Astrology 101 course. Then she uses a random digits table to choose 10 students from each of the twelve zodiac signs. For example, students born from March 21 until April 19 have the Aries zodiac sign, and Professor Gans chooses 10 Aries students at random. She uses a different part of the table of random digits to choose students having each sign. After obtaining her sample, Professor Gans has the selected students fill out a questionnaire. One of the questions in Professor Gans' questionnaire asks whether the student has ever given money to the American Nazi Party. Any students who have done so are likely to lie and say "No," rather than admit the truth. This is an example of what kind of error (explain)?