“Do you feel confident or not confident that the food available at most grocery stores is safe to eat?” When a Gallup poll asked this question, 87% of the sample said they were confident. Gallup announced the poll’s margin of error for 95% confidence as ±3 percentage points. Which of the following sources of error are included in this margin of error? Explain your answer.

a. Gallup dialed landline telephone numbers at random and so missed all people without landline phones, including people whose only phone is a cell phone.

b. Some people whose numbers were chosen never answered the phone in several calls or answered but refused to participate in the poll.

c. There is chance variation in the random selection of telephone numbers.