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Look, I made more charts! TL;DR: Good news! Fanfic completion rates are about 80% given enough time.
One of the most common reactions I saw to my original post was wailing and despair over the number of incomplete works on AO3. However, there's always the possibility that a lot of the unfinished works are actually WIPs that will be completed eventually, rather than abandoned works. To test this, I looked at rates of completion over time.
Fig. 1 shows the percent of multi-chapter fics that have been completed (listed by the year that they were last revised). I calculated this the same way I did in the first post, but broken down by year. The multi-chapter fics that were last updated in 2013 have a very low completion rate (32%), which makes sense. The ones last updated in 2012 have a 52% completion rate. 2011 fics jump to 69% complete, and prior to that, completion rate is around 80%.
The reason, then, for the low average completion rate, is that there are so many more fics posted in recent years. Fig. 2 shows the number of new fics on AO3 by year (both single-chapter and multi-chapter; non-cumulative -- 2012 is the set of only the fics last revised in 2012). I've also shown the projected total for 2013, if people keep producing new works at the rate they have for the year so far, and all the backdated fics, from before the founding of AO3, are listed in the "earlier" category. From this figure, we can see that people are posting lots more fics with each year that passes.
So, while the majority of recent multi-chapter fics are incomplete, take heart -- given enough time, most of them will probably be completed. (An alternate explanation is that people are getting much worse about finishing things as time progresses, but I suspect that that accounts for only a small part of the observed pattern, if at all. Time will tell.)