Which survey method is considered the least flexible?

Mail surveys are considered the least flexible method among the given options because they involve a predetermined set of questions that respondents must answer in a specified order without the possibility for real-time interaction. Once a mail survey is sent out, respondents can only provide answers based on what is physically provided on the survey form. If additional clarification is needed, or if a respondent has follow-up questions, there is no opportunity for immediate feedback or adaptability.

In contrast, phone and face-to-face surveys allow interviewers to ask follow-up questions, probe for deeper understanding, and clarify any misunderstandings right on the spot, while online surveys may offer some flexibility in terms of navigating through questions at the respondent's own pace. Mail surveys lack these interactive elements, making them less adaptable to the respondents' needs as they complete the survey.