Data source: June, 2011 (23 SE sites, not including Meta sites)

Question source: What can we learn about migrations?

Where do migrations originate, and where do they end up?

Sources (min 100)                Targets (min 250)

Stack Overflow    24082          SuperUser        12856
ServerFault        3747          Programmers       6681
SuperUser          3660          ServerFault       6678
Programmers         618          Stack Overflow    2286
Webmasters          350          Webmasters        1480
Web Applications    260          Web Applications  1145
                                 Unix and Linux     488
                                 Apple              401
                                 Android            327
                                 WordPress          276
  • The above results include deleted questions
  • People who have said that SuperUser is a dumping ground for miscellaneous computer-related questions certainly have a case. (Have you hugged a SuperUser mod today?) Stack Overflow accounted for about 9,500 of the migrations, with the bulk of the remainder from ServerFault (this isn't surprising, but I thought it would be useful to include).
  • While the data analysis did not specially consider the initial influx of questions to new SE sites, the most active sites in terms of migrations (say, Webmasters) also have a significant outflux of migrations as well, so IMO, while this does skew the stats a bit, it probably doesn't skew them as much as if those stats were run immediately after the initial influx. This would affect sites with lower absolute numbers, but those carry less weight anyway.

How well does a community know other communities? (Good outbound migrations)

Site                   Total Migrations    Good Migration %
Game Development                     16               100.0
Theoretical CS                       27               100.0
Unix and Linux                       44               100.0
Programmers                         618                98.9
Android                              83                98.8
Ask Ubuntu                           53                98.1
SuperUser                          3660                97.2
Web Applications                    260                96.5
Apple                                56                96.4
ServerFault                        3747                96.3
Webmasters                          350                95.7
Statistical Analysis                 71                94.4
Mathematics                          52                94.2
Gaming                               30                93.3
English                              29                93.1
Electrical Engineering               41                92.7
StackOverflow                     24082                90.3
Photography                          33                87.9
WordPress                            49                87.8
  • Results from sites with fewer than 10 migrations-out were discarded
  • A good migration is when a migrated question remains open on the target site, or is closed as a duplicate. In other words, "was the question a good fit for the target site?"
  • Network-wide, about 9 out of 10 migrated questions end up in the right spot.
  • Stack Overflow specifically had one of the lowest percentages and the highest volume -- increasing migration quorum was implemented as a result, so while that change is not reflected in these numbers, I suspect the good migration percentage from Stack Overflow has improved since then. Even so, 90% success is pretty astounding considering the number of people involved.

How well do other communities know our community? (Good inbound migrations)

Site                   Total Migrations    Good Migration %
English                              26               100.0
Game Development                    102               100.0
GIS                                  27               100.0
Unix and Linux                      488                99.6
Apple                               401                99.3
TeX - LaTeX                         235                99.2
StackOverflow                      2286                98.6
WordPress                           276                98.6
Mathematics                          95                96.8
Webmasters                         1480                96.6
Statistical Analysis                 83                96.4
ServerFault                        6678                95.8
Web Applications                   1145                94.8
Gaming                               59                93.2
SuperUser                         12856                92.8
Android                             327                89.9
Programmers                        6681                81.6
Electrical Engineering               48                79.2
Theoretical CS                       14                78.6
  • Results from sites with fewer than 10 migrations-in were discarded
  • I think the staggering drop-off below the ~90% threshold is due to ambiguity of a site's accepted range of questions. It took quite a long time even for me to figure out what is acceptable on Programmers. Unfortunately, there isn't a lot of other data to support that conclusion, but I know that the "Theoretical xyz" sites in particular had come under fire for this. (See this, unfortunately.)
  • Looking at the list of sites by name, I think we can conclude that good migration targets have a simple, clearly-defined range of acceptable questions. The less ambiguity, the better. This, of course, does not apply only to migrations, but probably to the site's overall success in general.

If you'd like to have a look at all statistics I collected for this question, including the full source/target migration matrix with close reason breakdown, you can download the Excel spreadsheet here.

  • 3
    Programmers has rejected 42% of Stack Overflow migrations (past 90 days), and I don't remember the reject rate being lower than 30%. And the quality of the successful migrations tends to be at the lower end of the spectrum (anecdotal). – yannis Apr 28 '12 at 22:31
  • 3
    Unfortunately, considering how wildly out of date your data is, it's hard to draw any conclusions from this analysis. Your estimates for Programmers, for example, are less than half the actual numbers. Every 10k user has access to their site's current migration statistics: not sure why you used a data dump from almost a year ago. @YahooAnswersenthusiast It was down to 23% last August, but has crept up ever since. – user149432 Apr 29 '12 at 0:18
  • @Mark: This was from an NDA dump, not publicly available. I'm sorry the data is pretty far out of date (I did the guts of this analysis quite a while ago); however, I think it's still valuable to be able to look at the trends and proportions rather than absolute figures. As far as Programmers goes, it's possible that the data is skewed by the initial migration, which as I said, I did not account for in the analysis. The low rank even at the point in time above does seem to correspond with what you're saying, though. – Jon Seigel Apr 29 '12 at 1:06
  • 3
    I want to hug random, but I'm afraid I might catch something. – Shog9 Apr 29 '12 at 1:35
  • You could try hugging one of the other mods :P – Ivo Flipse Apr 29 '12 at 18:25

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