In voting to migrate a question, I am allowed only three sites to which it might be migrated: math.meta.stackexchange.com, stats.stackexchange.com, physics.stackexchange.com

In particular, mathoverflow.net is not there. But I've seen questions migrated from mathoverflow.net to math.stackexchange.com.

Why only those three and no others?

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    $\begingroup$ The debate here is relevant. $\endgroup$ – Michael Greinecker Dec 12 '17 at 20:09
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    $\begingroup$ The cost of a bad migration is higher than the cost of not migrating a Question. Communities now have more specialized coverage than outsiders typically imagine when they propose a migration. My advice is not to propose a migration to site X.SE unless you actively participate in X.SE and know first hand that a Question would be welcome there. $\endgroup$ – hardmath Dec 12 '17 at 22:10
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    $\begingroup$ @hardmath : Your advice is as obvious as anything can be. But I'm wondering about the word "now". They now have more specialized coverage, as opposed to some other time when they didn't? $\qquad$ $\endgroup$ – Michael Hardy Dec 13 '17 at 0:22
  • $\begingroup$ MichaelH: Briefly, I'm thinking of the evolution of SE policies and mechanisms for migration. In the very early years it was possible for me to be active in at least two of the "big four" sites (Math.SE and StackOverflow) and be somewhat familiar with almost all of the sites. Change has come both in specialization of new sites recently and longer term in volume growth on larger sites (success, at some cost). I have in mind that now policies and resources are allocated appropriately to these changes, so arbitrary migration paths are pruned out. More if you wish, as an Answer. $\endgroup$ – hardmath Dec 13 '17 at 17:11

The number of migration targets available for users is restricted, mostly to avoid incorrect migrations, as such migrations are difficult to undo and also impact another community.

Moderators can migrate to any site in the network. If you want to migrate somewhere else flag for moderator attention (flag, other) and suggest it.

The exact targets available for migration are chosen by SE, informed by precedents and discussions with the respective communities. If you notice frequent need for migrations to a particular other site, feel free to propose it. Usually, sites that are still in beta are excluded. Note though that the total number usually does not exceed four, thus the list will stay short.

The path to MO is indeed asymmetric, which is unusual. I am not certain why the path to MO is asymmetric. Michael Greinecker provided some context that recalls concern over migrations to MO in this community, another factor might be concern on MO over incorrect migrations.

But sometimes questions also get migrated from here to MO (by a mod). Indeed it is the most frequent target site (with twelve migrations over three months). You can find some data about migrations on the respective tools page (link available to 10k+ users).

As one can see from that page overall migrations are pretty rare, e.g., there were only six over the last three months to the statistics site, and around 35 total. Thus, to require moderator involvement is not a problem.

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    $\begingroup$ I hadn't realized this couldn't be done freely, doubtless because I'm a virgin at such things. Probably more than a dozen times I've seen questions posted to m.s.e. that looked as if they would be very far over the heads of $99.99\%$ of participants in m.s.e., and that I could not answer, but would fit perfectly at stats.stackexchange.com. I've responded to those by posting a comment under them suggesting they post there. $\endgroup$ – Michael Hardy Dec 12 '17 at 20:37
  • $\begingroup$ This is also a good way to proceed, especially as long as there is no substantive content beyond the question. $\endgroup$ – quid Dec 12 '17 at 20:46

Questions should only be migrated if they are both (a) off-topic on the originating site and (b) on topic on the receiving site.

There are categorically no questions that are on-topic on MO and off-topic here, so it is never appropriate to migrate a question from here to MO.

This strikes me as an entirely reasonable reason not to offer the technical option of performing such an erroneous migration.

  • $\begingroup$ Upon what do you base your first sentence? Is this an official policy? $\endgroup$ – Michael Hardy Dec 17 '17 at 19:32
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    $\begingroup$ @MichaelHardy: I base that on ordinary common sense. $\endgroup$ – hmakholm left over Monica Dec 17 '17 at 19:33
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    $\begingroup$ How does common sense forbid migrating a question to a site where it fits substantially better? As I said, probably more than a dozen times I've seen questions posted to m.s.e. that looked as if they would be very far over the heads of $99.99\%$ of participants in m.s.e., and that I could not answer, but would fit perfectly at stats.stackexchange.com \qquad $\endgroup$ – Michael Hardy Dec 17 '17 at 19:35
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    $\begingroup$ @Micael, the asker of such questions is free to post them somewhere else if they want, but if they choose to post them here and they are on topic here, it is nobody's business to say that users here should be forbidden from posting answers to them. $\endgroup$ – hmakholm left over Monica Dec 17 '17 at 19:42
  • $\begingroup$ But I was contemplating migrating one of my own questions. $\endgroup$ – Michael Hardy Dec 17 '17 at 19:57
  • $\begingroup$ @MichaelHardy: Ah, that's different then. I thought we were talking about standard migration-by-five-close-votes without the asker's consent. You can raise a custom flag on your question and ask for a moderator to migrate it. $\endgroup$ – hmakholm left over Monica Dec 17 '17 at 20:17
  • $\begingroup$ @MichaelHardy It seems to be an unofficial policy. There are questions posted on sites that I feel would get better answers on other sites (and in some cases already have), but are perfectly on-topic where they are. $\endgroup$ – wizzwizz4 Dec 20 '17 at 19:00
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    $\begingroup$ Plus, in order to migrate you have to go through close -> off-topic -> belongs on another site, so it's semi-official policy. $\endgroup$ – wizzwizz4 Dec 20 '17 at 19:01

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