Government agencies, sociologists and marketing agencies use self-completion questionnaires to collect information from the general population and consumer groups. Typical advantages of this type of survey are typically low costs and that respondents appreciate anonymity. On the other hand, there is no guarantee that enough surveys will be filled in to form an accurate view of the research group.
Self-completion questionnaires are the cheapest method of collecting information from large numbers of people. This is a key advantage for researchers, especially in the case of government surveys, such as a census, when information is gathered across the entire nation. Clearly, there are costs -- for printing, postage and collation of the completed questionnaires -- but these are much lower than the cost of paying interviewers to help respondents fill in questionnaires.
Anonymity is an advantage for both researchers and respondents. This is especially true if the research topic is of a sensitive nature. Respondents are more likely to answer honestly if they don't have to reveal identifying information. Postal surveys offer respondents a better guarantee of privacy contrasted with online surveys, where respondents are traceable. Another advantage of self-completion surveys is that the researcher can't influence the respondents' answers by using her voice or facial expression to imply that a particular answer is the "right" one. Therefore, survey results won´t show interviewer bias.
One of the most significant disadvantages is the lack of control over response rate, which can be as low as 20 percent. Unless respondents are offered an incentive to answer questions and return the survey, such as being entered into a prize drawing, questionnaires can end up in the garbage with unsolicited mail. Receiving questionnaires after the completion deadline is another issue that makes this research method problematic, especially if time is an issue.
The researcher has no control over who fills in the questionnaire, or whether any attempt is made to answer questions accurately as opposed to ticking boxes at random. Anyone at the mailing address may have picked the form up and filled it in. This potentially makes it more difficult for the researcher to get a true impression of a particular age group or gender.