One of the most common reactions I saw to my original post was wailing and despair over the number of incomplete works on AO3 -- I reported that only 51.1% of multi-chapter fanworks on AO3 were marked complete. 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%.
One 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.)
[Another explanation that has occurred to me since posting this: possibly lots of authors are archiving their older works on AO3 -- but only the completed ones. AO3 is relatively young, so there may have been a lot of that happening in the past few years. I expect that also doesn't account for a huge percentage of the fanworks on AO3, but I don't know for certain.]
Edit: a note on data collection
I wanted to analyze just multi-chapter works and their completion rates on AO3, since single chapter works are inherently complete. I used AO3 Works Search to look at both number of works marked Complete and number of single chapter works. I did so year by year, using the date fields. I discussed my methods for combining this data to get completion rate in Tumblr post I haven't yet archived:
For “Number of chapters”, there’s no way to search for multi-chapter works; I searched for single-chapter works only and then subtracted that from total number of works.
For the completion chart, I was only interested in multi-chapter fics, since single-chapter works are inherently completed. So, first I found the total number of single and multi-chapter works (as above). Then (number of complete works) - (number of single-chapter works) = (number of complete & multi-chapter works). And subtract that from the total number of multi-chapter works to get the incomplete multi-chapter works.