Questionnaires mainly give out quantitative. As this is quantitative data being given this will facts and numbers that are precise and have a high reliability. A questionnaire could be conducted by a positivist sociologist that want to identify the underlying causes of people's behaviour, in particular, closed questionnaires.
Self-completion questionnaires are less time-consuming for researchers and would be considered the better option over interviews as they do require the researcher having to find the time to go speak to that person face-to-face. As an advantage of conducting self-completion questionnaires they are anonymous. If the researcher has included any embarrassing (EG. Sexual or illegal acts) by giving the anonymous option allows the researcher to gain that information from people as they are mostly to tell the truth that way. These are also easy to distribute depending on what the researcher wants to find out. To discover the rule of linguistic deprivation in educational underachievement the researcher could easily distribute them throughout a school. With questionnaires the questions given are unlikely to be biased therefore information that is received will be valid and reliable.
Unfortunately, many people cannot be bothered to spend their time answering questionnaires, especially if they are pages long, in addition to this, the response rate is likely to be low rendering the survey pointless. When attempting to investigate the role of linguistic deprivation in educational underachievement, self-completion could prove to be a restriction if the questionnaire is lacking simplistic language then it would be harder for the participant to answer the questions. If the participant speaks little English then there would be no possibility for completion. It is also difficult to go into depth when it comes to questionnaires. Yes it may be obvious to see where the underachievement is, but what is the key factor of their deprivation. In addition if these questionnaires are done within school, they are most likely to either find out what they're friends have written and copy that down if they have no clue what to answer or even ask a teacher what a word means or how to answer it. A way to prevent this is for the researcher to add 'check questions', this is where they repeat a question but rephrase it.