Every poll involves a questionnaire that contains a standardized set of questions that are asked of every person. The way a question is asked can affect the answers that people give.
In an open-ended question, people answer in their own terms. In a closed-ended format, people choose from a given list of answers. (The vast majority of polling questions are closed-ended.)
One example is the “most important problem” question. This is asked most commonly in the open-ended form used by the Gallup organization: What is the most important problem facing the country today?