A questionnaire is considered to be the written document used to obtain information from respondents. It consists of a predetermined set of questions used to collect data (clinical data, social status, etc.). There are two main types of questionnaires: structured (closed-ended, standardized, formal) and unstructured (unstandardized, informal). Closed-ended questions will refer to those in which all possible answers to a given question are listed on the questionnaire, whereas the term open-ended will apply to questions in which the possible answers are not listed in advance. The three common ways of obtaining information are: by sending a questionnaire by mail to individual to fill out and return, by having an interview by phone or in person. Sometimes combinations of these methods are used.