Stack Exchange is a hub of vertical (domain-specific) question and answer sites (including programming, system administration, , theoretical computer science, physics, gardening, English, and other subjects) using the eponymous software. The pioneering site for the software was Stack Overflow. Stack Exchange’s key features include: specific questions with the intent of getting specific answers, voting on questions, voting on answers, concept of an accepted answer determined by the question-asker, reputation and gaming features based on upvotes received, and rearranging the order of appearance of answers in a page based on votes. Specific questions tend to get answered accurately and quickly (often within minutes), usually by “regulars” who visit the site hourly and have accumulated huge reputations. Stack Exchange’s site statistics as well as question answering rate are impressive (2010 summary).
Math Stack Exchange is one such site, meant for undergraduate level mathematics questions and answers (this differs from the more research-level question-and-answer site Math Overflow).
Some of my random thoughts on the relation between Q&A sites and reference sites (like Wikipedia or the subject wikis):
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The success of Q&A sites, particularly those of the Stack Exchange type, often relies on the fact that reference sites either don’t have sufficiently deep information, or that such information is not easily accessible or navigable for many users. For instance, I’ve seen questions asked on Math Stack Exchange in group theory and two of my common reactions are: (i) Oh, this is content I’m planning to put up in Groupprops in a few weeks/months! and (ii) Oh, it’s already there on Groupprops, but somewhat hard to find. See for instance this question. If somebody has not yet already posted an answer, I post a link, but generally some enterprising soul has posted an answer already — often going to quite some effort to type out a laborious answer.
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It’s overall preferable to have a reference site that automatically and quickly answers your question than to have to type it to a Q&A site and have somebody else type an answer. Apart from saving time, the reference site can go into greater depth, have greater reliability, link to related material, and answer questions at the periphery of your consciousness. Q&A is, for many purposes, an inferior good.
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One goal of a good reference site is to push out the boundaries (i.e., raise the standards) of the questions that people consider asking on Q&A sites. The “Wikipedia test” (respectively, “Google test”) filters out any question that can be easily answered by Wikipedia (respectively, Google). People who ask questions that fail these tests risk their reputations. The better Wikipedia and Google get, the more the questions filtered out. With the subject wikis, I am hoping that yet more questions get filtered out because they can be immediately answered by search/lookup.
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One advantage of the Q&A style as a reference — the focus on questions creates a somewhat different hierarchy of importance and relevance, which might be more useful for somebody with a question (or the beginnings of one) to locate it. Also, the Q&A navigation experience is more suited for some moods and needs. However, there’s an alternative to Stack Exchange-style active Q&A: “passive Q&A”. This is Q&A lists deeply embedded in a reference — with short questions and somewhat longer answers that link deeply to content in the reference. A prototype is Questions on normal subgroup. Due to the semantic richness of the underlying reference layer (Groupprops), the questions can be organized quite well and the whole experience can be very smooth.
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Spontaneous questions asked by real people will still keep uncovering new areas of confusion and scope for clarification that passive Q&A has not yet covered. Passive Q&A must regularly borrow new insights from gaming Q&A to continually improve.
The observations above pertain strictly to Stack Exchange style sites. There are other Q&A sites, such as Quora, that are hinged more on exploration, discovery, and creating new knowledge — something that cannot be included in a reference because it needs discussion and opinion-soliciting to develop in the first place. Quora aims to use the question and answer model to create new knowledge; according to their about page, Quora is a “continually improving collection of questions and answers created, edited, and organized by everyone who uses it. The most important thing is to have each question page become the best possible resource for someone who wants to know about the question.” Here are Seb Paquet’s thoughts on Quora and how it differs from a reference.
For more on the similarities and differences between Quora and Stack Exchange, see this Quora question and the Quora vs Stack Exchange topic on Quora.