Kaveh asked what problem the two level split into research and non-research sites would cause. This has now gotten a couple of answers. I asked what problem is solved which I think is more important, and Kaveh thought this would be a different discussion, so here it is.

The only argument I've heard is that you need separate research sites to not waste the time of experts, but that is a strange statement, answering questions is almost by definition time wasting, you do it because you enjoy it, not because you have to or make money from it.

And in any case experts generally do not have the time to sit and pounce on questions when they arrive (although there are some experts like that on Stackoverflow too) instead you will check very narrow special interest tags, or you will look for questions with bounties or questions other people haven't been able to answer. Simple questions will be answered by others.

So the claim that an all-inclusive site would waste the time of experts is clearly incorrect. Each expert chooses how much of his time and one what he wants to waste. The existence of simple questions does not force anyone to answer them.

So: What problems does the two-level split solve?


We have been discussing this for a while now in several threads, and I think we are starting to run out of steam. My conclusion is this:

The problem a two-level split solves is that some researchers doesn't want to hang on a generic site. The arguments for why are to a large part clearly just rationalizations of their feeling on the subject, but some arguments make sense, like that the research-level questions tend to be more discussions than answers, and hence are a different type of questions. But rational or not, the opinions of the researchers should be acknowledged.

In my opinion it would have been better to start with a generic site, and split out a research-level site if there seems to be a demand for this (ie research level questions), but not enough supply (ie answers), although this is of course too late with cstheory.SE.

  • StackExchange main page currently lists: StackOverflow, Programmers, Game Development, Wordpress, Code Review (Beta). So please stop stating SO as example of an "one for all" community already.
    – Raphael
    Commented Feb 11, 2011 at 10:52
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    @Raphel: Huh? These sites are not split up depending on how advanced the questions are. It's different focus areas. Stackoverflow embraces everyone from newbies to gurus. That is a fact. There is no split into different levels of complexity. Commented Feb 11, 2011 at 11:20
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    That might be on the label, but the generalistic approach does apparently not suit the needs of a significant amount of people. Why would you expect it to work for (t)cs?
    – Raphael
    Commented Feb 11, 2011 at 11:38
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    @Raphael: Again, it's not a split on level, it's a split on topics. SO is a one for all community. That is a fact. There is no site for "advanced" programming or "research-level" programming. Commented Feb 11, 2011 at 11:44
  • One for all != Best for all. Anyway, the fact that cstheory graduated from Beta proves that there is a need, and a community to drive it.
    – Raphael
    Commented Feb 11, 2011 at 11:51
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    More to the point, the fact that there is cstheory but no general CS and no novice TCS site suggests that cstheory precisely fulfills certain needs.
    – Raphael
    Commented Feb 11, 2011 at 12:14
  • Here is a link to the other question: Two level model (MO/Math.SE) vs one level model (SO)
    – Kaveh
    Commented Feb 11, 2011 at 12:52
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    Nobody opposes a split into different subjects if there is enough interest in that. This is a discussion about a split into how advanced the users are. Trying to claim that this is the same as splitting up per subjects is nonsense. I don't know why there is no cstheory site that encompasses everyone, but using this non-existance as an argument for the two-level split doesn't hold up. Rather a general site should have been started first, and a research-level site should have been split out of it if needed. That hasn't happened, so we can't use this as evidence that it is needed. Commented Feb 11, 2011 at 13:36

8 Answers 8


My impression is that a high proportion of the highest rep. MO users are willing to participate in maths.sx, but a much lower proportion of mid rep. users. I saw that 9 of the top 10 rep. MO users are 2k+ users on maths.sx, while from the first column (7 users, rep. 3.1k to 3.4k) on page 5, only 4 users have maths.sx accounts at all, and all but one are under 500 rep.

This suggests that the most motivated users want to put their energy into making both sites work, but that there are many experts who are just interested in an experts-only site. Thus I claim a pure experts' site catering to broad academic discipline that has a need for a non-subjective Q&A, like maths and physics, will do better at attracting experts than the general sites.

Incidentally, do note that most academics split time between teaching and research, and to them the sites follow the same distinction: the one is for expert researchers, the other is for expert university-level teachers, who largely, but not wholly, are the same people, but which serve completely different activities. Aaro suggests that the distinction between advanced and beginner-level is soft. I think the distinction between teaching- and research level is pretty sharp: I guess that the number of questions on MO that are "controversial" (i.e., there is serious dispute about whether they are research level) among 1k+ rep. users is a very small fraction of the total.

  • Oddly, I find your argument seems to go against you: yes research and teaching are different activities, but nearly all universities recognize the value of putting them under the same roof.
    – zergylord
    Commented Jul 21, 2011 at 19:00
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    @zergy: The points I made suggest that there is much more interest on the part of professional researchers for research-only sites than for combined research and teaching sites. Universities are different from Q&A sites in that they pay salaries and provide a context where research groups can be formed. Even so, academics typically try to reduce their teaching load to concentrate on research. Commented Jul 22, 2011 at 7:27

The research/non-research split is mostly not a split on topic, but it's not really a split on level either. It's more of a split on the target audience: researchers in X form a very distinctive subcommunity amongst the wider community of people interested in X. Charles Stewart cites some statistics on cross-participation between MO and math.se; my interpretation is that there are researchers who spend a lot of time on discussion sites, and they frequent both sites; while other researchers spend little time, and they stick to MO where the content that's most interesting to them is easy to find.

The split is also a split on content, even if the distinction is not the topic per se. Most of the content of MO is not interesting to non-researchers because it is incomprehensible; and most of the content of Math.SE is not very interesting to your typical math researcher because it's not advanced enough. It's easy for non-researchers to ignore the questions they don't understand; it's a lot harder for the researcher in a hurry to find the gem amongst the boring (to him, not in the absolute!) lower-level questions. (No, tags help pick questions based on their topic, but not based on what I would call their flavor.)

There are precedents for sites with overlapping topics on Stack Exchange:

  • Server Fault vs Super User: a lot of system administration is on-topic on both sites. SF is specifically for professionals who share common prerequisites and interests; SU is more generalist and caters to amateurs.
  • Ask Ubuntu vs Unix SE: all the questions on AU could be asked on Unix.SE. But they would (and do) get different answers, because the sites cater to different communities (AU primarily to people who care specifically about Ubuntu, Unix.SE to more general tinkerers).

TL,DR: the two-level split solves the problem of different audiences expecting different content. And there are precedents that work.


Two levels serve the same purpose as splitting by topic: focus.

High-level questions need careful consideration and answers by experts. Expecially in TCS, the number of peers is generally low. A lifespan of hours as on SO is not sufficient for that; our questions are active for days.

Low-level questions need active users with the drive to look through a lot of questions and (ideally) superior knowledge. Apparently, not all experts see themselved in that role.

Also, I guess that a different kind of answer can be and is expected. On research level, there likely is no short (read: a few sentences) answer if any at all. References to published work and/or a long, iterative answering process has become typical on cstheory. There is usually no need for precision; the exchange of ideas and hunches if often sufficient amongst peers.

On the other hand, questions on lower levels can often be answered very shortly, but might need to be precise wrt to a certain notation and have a didactic finish in order to help the novice.

As you can see, even though the topic does not change between levels, the needs and preferences of users do.

PS: Note that I myself have long struggled with this question. I have come to accept that important users do not like to be part of a larger community. We do not have to like that, but it is fact. Therefore, a merge would not benefit the research community, which is our goal. However, other users would like to help beginners, too, and are likely to frequent such a site had we one.

  • 1
    Questions on SO are active for days too, if needed. Sure, most questions can be answered quicker than that, but not all. And yes, not all experts see themselves in the role of looking through a lot of questions. But how is that a problem? I don't generally do that (except if I'm bored) on SO either. And these are not distinct types of questions, it's a spectrum, some are very fast to answer, some very long, some require discussion, some doesn't, and most are somewhere inbetween. So this answer doesn't explain why this can't co-exist on one site. Commented Feb 11, 2011 at 11:18
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    I honestly do not think you can compare the spectrum from SO to that inside a science community. Anyway, answer quality on SO tends to be quite bad for non-mainstream languages, so I really, really do not want to adapt whatever goes wrong there.
    – Raphael
    Commented Feb 11, 2011 at 11:23
  • Well, I do think you can compare them. The problem still is that you can't actually explain what the problem is that the split solves. The answer quality is bad on topics where there aren't enough users to answer them. Making a site with fewer users isn't likely to solve that, is it? Commented Feb 11, 2011 at 11:29
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    The last paragraph here is critical. Stack Exchange's mission is, loosely speaking, to "make the internet a better place." Obviously, it's also a business, and although few of us know the details yet, they clearly have long-term plans to monetize it. So, perhaps it is true that a merge would not benefit the research community, at least as they perceive it; the more pertinent question is, what are the practical benefits of a skill-level segregation to either community or the internet? Researchers' personal feelings are not really a practical benefit, and certainly not a measurable one.
    – Aaronaught
    Commented Feb 11, 2011 at 18:42
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    As far as I understand, a new SE site is created if and only if there is sufficient community support. This has proven to be the case for cstheory. Unknown longterm plans of our hosts or global considerations can clearly not be taken into account. So, assume the process went through for a beginners/general CS site. Then, by definition of the process as I know it, there is the need for two communities. I the criteria that make up the founding system are not what is intended, the process has to be changed.
    – Raphael
    Commented Feb 11, 2011 at 19:18
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    Raphael, the fact that it doesn't work that way is precisely why this entire discussion area was created. Area 51 is not a pure democracy, the owners have creative control and veto power, and concerned members are allowed to voice their concerns to them (and the rest of the community). Sites are only created once there is sufficient support, but the reverse is not necessarily true - simply because people want a site to exist does not mean that it belongs as its own independent site, or that it belongs anywhere at all. And none of that is germane to the question being asked here, anyway.
    – Aaronaught
    Commented Feb 11, 2011 at 23:03
  • @Aaronaught: I'm not sure researchers personal feelings isn't a practical benefit. A site with loads of researchers could attract more job advertising than a general site for example. I do think the only viable answer here to what problems the two-level split solves is exactly that: A one level site might drive away researchers. We don't know if it would, really, but it seems likely that it would, simply because many researchers themselves say that they would leave. They have no rational reason for this, but you can't ignore reality because it's being irrational. :) Commented Feb 12, 2011 at 9:47
  • @Lennart: What I'm actually seeing is researchers saying that other people would leave. So far no people, or very few people have actually come forward to state that as a certainty, and unfortunately the attitude that's been taken by some of the people in favour of the split has poisoned the objectivity of the conversation, so we really can't know for sure how likely it is. We're weighing the absolute certainty that an elitist site would drain audience from an all-encompassing site against the unverified possibility that the all-encompassing site could experience more attrition.
    – Aaronaught
    Commented Feb 12, 2011 at 21:39
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    @Aaronaught: I dispute your claim that "an elitist site necessarily drains audience from generalist site". This seems exactly as unverified as the second claim. In fact, Charles Stewart (above, in discuss.area51.stackexchange.com/questions/630/…) provided verifiable evidence that the opposite may be true, that an elitist site attracts people who would not be interested in a generalist site; in the long term this may benefit the generalist site. Commented Apr 7, 2011 at 10:58

A part that was not mentioned is that a research level site also helps in learning for research level students. When you are a graduate student, one of the things you need to learn (apart from all the difficult technical knowledge of your field) is the softer skills of what kind of questions and presentations are interesting to your peers. When I visit MO or cstheory I am able to get a flavor of what researchers find interesting. If there is a general site (say math.SE), then although I can ask my questions on it, by browsing the site I don't get a feel for what researchers find interesting.

When I am browsing cstheory, I learn something from reading and answering other's questions. If I go to more general sites then I learn only from my own questions. Thus, the two level model has a chance to increase the amount of effort experts will invest in browsing and answering questions. The reason researchers seem to prefer research-level sites is not snobbery or a disdain for the non-researcher: it is the fact that like any person they have limited time and energy.


From the point of view of the research level site there's basically only one reason: to have a good site you need many researchers (because there are still many subfields so you need to increase the probability that you'll find your peer and ideally more of them to start a discussion). There are two ways to achieve that: a) SO way of being huge and that means even relatively low count of experts translates into big enough absolute number; or b) just try to collect the experts somehow. I am not sure how good 'a' is for doing research (is there any research going on at SO?) but 'b' definitely works.

There is also a secondary reason of lot of experts not wanting to be a part of low level community. Again, 'b' solves this.

  • I guess you would answer "No". ;) There is loads of research going on, but not in a way you would recognize, as any university type research into programming would be directed to the theoretical computer science department. ;-) SO is a fundamentally practical site, but there are discussions both about very theoretically tricky issues, as well as about things like whether the OO models ideas of protection really makes sense, etc. In some sense, all programming where you are trying to do something that hasn't already been done is "practical research". Commented Feb 12, 2011 at 9:55
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    @Lennart: indeed, part of the confusion stems from what research means. I don't consider the usual problem solving and thinking to be research. That's the daily bread of basically every living person. But professional research is something completely different. It's a job in itself, people are writing papers, etc. Most of the normal people don't do that :)
    – Marek
    Commented Feb 12, 2011 at 10:06
  • @Marek: Right, and in practical use, that just doesn't happen. You wouldn't find materials research on a site for home improvement either. You'll find "practical" research, i.e "I tried doing X and Y happened". That means that a research-level SO doesn't make sense. But it doesn't automatically mean that a research-level cstheory makes sense either, so the lack of research in the common sense on SO doesn't say anything either way, unfortunately. Commented Feb 12, 2011 at 10:14
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    @Lennart: "and in practical use, that just doesn't happen" -> yes and this is the very reason why people want to create separate research sites, isn't it? You probably won't get research (in the professional sense) at usual sites. Of course, I am not saying it's impossible; there has been no evidence that SO can't work as a research-level site too.
    – Marek
    Commented Feb 12, 2011 at 10:19
  • @Marek: "Usual" sites? Nonsense, I said practical use, not usual sites. You won't get academic research on practical sites, like SO or home improvement or gardening. That doesn't mean you couldn't have it on theoretical sites like cstheory. Maybe they still should be separate, but as I clearly stated above (please read before you answer), the lack of research on SO doesn't mean you can't have research on a generic cstheory site. Commented Feb 12, 2011 at 10:25
  • @Lennart: I don't understand what you are arguing about now. Of course you can have it at sites aimed at research (MO, CST, TP, etc.) because it's the very point why those sites were/are created...
    – Marek
    Commented Feb 12, 2011 at 10:41
  • @Marek: I didn't even mention sites aimed at research. Did you misread my comment again? Commented Feb 12, 2011 at 11:00
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    @Lennart: no, but perhaps you misread my answer and all my comments because research sites are what I am talking about all the time... In any case, this discussion seems to be rather pointless.
    – Marek
    Commented Feb 12, 2011 at 11:13
  • @Marek: The discussion is what problems a two-level split into research sites and general sites solves. That discussion, per definition, includes not only research sites, but also general sites. I'm sure you understand this. Commented Feb 12, 2011 at 12:10
  • @Lennart: sure. What I don't understand is how these two sentences of yours make sense together: "I didn't even mention sites aimed at research", "That doesn't mean you couldn't have it on theoretical sites like cstheory". Are you implying that CST is not a research site?
    – Marek
    Commented Feb 12, 2011 at 13:01
  • @Marek: I am stating that there is a difference between practical topics, like programming and theoretical topics, like CST. The lack of academic research on general practical sites does not imply that general theoretical sites could not host academic research. This I have now stated several times. I do not believe that I am in any way unclear about this. You should try to adopt an attitude where you attempt to understand instead of intentionally try to misunderstand. Commented Feb 12, 2011 at 15:50
  • @Lennart: okay, I can honestly say that I don't understand you at all now, try as I might...
    – Marek
    Commented Feb 12, 2011 at 15:56
  • @Marek: You asked if research happened on SO. I answered that it doesn't, but that's not because it's a general site, it's because it is a practical site. It is hence not an argument against general sites. I really do not think there is any human way to say it any clearer, and I've now said it four times in a slightly different ways, so I'm giving up now. Commented Feb 12, 2011 at 16:31
  • @Lennart: I also don't like to repeat myself but I've already said: "there has been no evidence that SO can't work as a research-level site too" and I was the first to state this. So why are you arguing about point that I agree with you about? The only explanation possible is that you actually don't read my comments...
    – Marek
    Commented Feb 12, 2011 at 16:37
  • @Marek: I have in no way contradicted or argued with that statement. You seem to be talking to some sort of straw man instead of me, which explains why it's so hard for me to make myself understood. I'm giving up now. Commented Feb 12, 2011 at 22:16

I do not know exactly which problems the two-level split solves. I can write my guess, but I am not sure if it is of any use. Not only that. I do not know whether two is the best number or not. Maybe one is better, maybe three is better. Why two? I do not know.

But why should I even care? The split between Math Overflow and Math Stack Exchange seems to be working. I am not interested in sociological study about why the two-level split works. The only important fact for me is that it seems to work. It is only reasonable to imitate what is working now. When I think about Stack Exchange for TCS, it is the most natural to imitate MathOverflow and Math Stack Exchange than, say, Stack Overflow.

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    Note that the split between MO and math.SE is slightly fictional. math.SE is a general site. Research level questions are not off topic. MO is not run by the SE people. We don't have any example of an actual split. If cstheory should imitate math.SE/SO, then cstheory should be a generic site, and those who doesn't want to participate in a generic site has to go off and start their own outside of the SE network. In practice, saying we should imitate this split is saying that SE should not have two-level splits. ;) Commented Feb 12, 2011 at 9:40
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    @Lennart: You are taking it for granted that the fact that MathOverflow is not run by Stack Overflow Internet Services is an important factor. If that factor is necessary for success, then the two-level proposal of cstheory will probably fail. But we do not have any evidence that it should fail. That is why the proposal should be reopened. Basically you are just pulling our the legs of the people who are trying to see how things turn out by blindly believing every factor which may work against the success and blindly ignoring every factor which may contribute to the success. Please stop. Commented Feb 12, 2011 at 9:45
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    @Tsuyoshi Ito: It is an important factor, because the fact is that math.SE is not a low-level site, it's a general site, and questions can not be migrated. Again: If you want to follow that model, that means cstheory should be a general site. I don't understand your claim that this means cstheory should fail, and your accusations against me are complete nonsense. I don't believe or do any of the things you claim. Commented Feb 12, 2011 at 10:10
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    @Lennart: Do not decide which part is essential for success. You do not know, I do not know, no one knows. I am only claiming that the two-level split between research level and general level has reasonable chance to be successful (maybe 10-50%?). I am not claiming that it will definitely work, I am not even claiming that it will probably work. You are claiming that it will not work based on something no one knows for sure. Just stop that and give them a chance. Commented Feb 12, 2011 at 10:14
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    @Tsuyoshi: I have never decided nor ever claimed to decided or even had an opinion on, which part is essential for success. Your are arguing against a completely fictional standpoint which I do not have nor have ever had or expressed. I have never said that "it" won't work, and that's even considering that I have no idea what you mean with "it". Stop what? Give who a chance? Commented Feb 12, 2011 at 11:02

The two-level split is an attempt to solve the problem of SE sites not having a "difficulty level" filter. Advanced questions about a particular topic are in many ways like a separate topic from trivial questions about the "same" topic. And while SE sites allow for filtering of topics, there is no way to distinguish between the "advanced" version of a topic and its "beginner" counterpart.

In the past, some proposals have been made on SO meta about adding a "difficulty level" to questions. These proposals have always failed because nobody has ever suggested a good way to implement such a feature. Usually, it's because the proposal was to allow the person asking the question to rate the difficulty of the question -- but I don't think this would ever work well (and apparently most others agreed, hence the proposals always have failed).

The two-level split that we see with some sites is, IMO, simply an implementation of this same "difficulty level" feature, albeit not necessarily a great way to implement this feature. I would propose that, in addition to the two voting buttons, each question on an SE site should have two additional buttons: "Too hard" and "Too easy". Experts who don't want to bother answering a question because it is too basic, simply click "too easy". Others who don't know the answer to a question because it is beyond their level of expertise click on the "too hard" button. This way the community decides what the difficulty level of a particular question is.

There would also need to be a mechanism for filtering by difficulty in addition to filtering by topic. This would solve the same problem solved by the "two-level" system, but IMHO, in a much cleaner way.

In any case, no matter how this problem is solved, I think a solution for it is desperately needed. Certain SE sites (SO, in particular) are becoming so full of very basic questions that they are beginning to fail as a good resource for experts seeking answers to difficult questions.

  • I don't think that this is enough. I don't think anyone here would give an answer for an explicit ruler and compass construction of a regular polygon for the highest known Fermat prime. It's not easy, it is not an interesting research problem, either. The point of MO is that it filters questions that are interesting to research mathematicians.
    – user11235
    Commented Oct 10, 2011 at 15:03

The point of the split is that people can decide how to divide their time between the sites.

You will find a lot of experts who are not willing to wade through 95% questions that are uninteresting by level, because of the remaining 5% questions still only a small percentage will be pertinent.

On the other hand, if they can get what they need from the research site, they might well decide to pass 10% of the time on the general level site as opposed to 0% on the frustrating general site. And researchers who pass 10% of the time on the general level site will have a higher reputation there than on the other site.

You have to accept that a researcher very rarely learns mathematics from answering undergraduate questions, many people with high rep on math.SE see their few own questions go unanswered.

And no, tags don't do the same thing as a split. On math overflow the questions on the frontpage indicate clearly that a standard undergraduate question is not on the same level, no information like this is conveyed when posters here (mis)use subject tags.

Level tags are not allowed here and posters would not use them correctly. This is not just about blocking basic questions. If I am not interested in research in set theory, say, I cannot block the corresponding questions here without being unable to see undergraduate questions on de Morgan's rule I could and would answer.

I came here to math.SE BECAUSE I first heard about MO and then heard about math.SE there.

  • +1 good points on human behaviour. Nonetheless i dont think theophsy would have gathered so much expters on high level without physics.SE before it. Sites like philosophy.SE dont work at all, they are thematically to isolated and can attract no experts, as not much existing on SE for this topic. experts sites like bio evolution or cogn. neuroscience will implode like philo.SE, as no experts here. Imho they should forbid experts site for topic too isolated, because it damages the quality in a lasting way. No phil. student will register on current phil.SE site Commented Oct 10, 2011 at 22:42

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