Updated to include the last two quarters of 2012.
I think what's most interesting about the update is how the numbers have changed over time. For instance, when I originally answered this question quarter 1, 2012, was complete. There were 409,490 undeleted, open, questions that quarter of which 85.61% were answered and 70.71% answered "well". Those numbers today are 405,131 questions, 87.47% answered and 74.30% answered "well", which implies that as time goes by the Stack Overflow community is doing something about the older answered questions, just not very much. The increase in the number of questions with a score of 1 or more also indicates that SO users are upvoting (viewing/using?) older questions, which is good.
The extremely noticeable consistency between the current and previous results is the fact that all these downward trends have continued, things are getting worse. On current trends, quarter 3 2013 will have less than 50% of questions where an answer is either upvoted or accepted. If the trend in answering/deleting older questions continues then it'll be rescued later but the same will happen again, permanently, around quarter 1 2014.
This is a "problem" that needs to be fixed, somehow.
I know this isn't an answer but it's far too long for a comment. There's a problem with your SQL, caused by the LEFT OUTER JOIN; you're counting questions multiple times so the problem is both slightly better and slightly worse than what you think.
I also disagree with your definition of a "good" answer. By definition an accepted answer is "good" as it has helped the OP (unless of course they've been pushed into accepting it by loads of comments but that's another matter). I've excluded Community owned questions and answers as I don't really think they are relevant (and it helps the query to work!).
Your first set of results on the number of open questions by quarter now returns the following:
Year Quarter Questions Answered GoodAnswer ---- ------- --------- -------- ---------- 2008 3 17508 99.99% 99.83% 2008 4 38790 99.87% 98.11% 2009 1 53441 99.73% 96.23% 2009 2 75339 99.56% 94.24% 2009 3 98426 99.22% 92.49% 2009 4 113136 99.02% 91.72% 2010 1 142909 98.62% 90.76% 2010 2 159213 98.01% 88.50% 2010 3 187222 97.56% 87.32% 2010 4 205903 97.41% 86.87% 2011 1 267895 96.98% 85.83% 2011 2 298140 96.16% 84.30% 2011 3 312239 95.30% 82.52% 2011 4 316593 94.46% 81.55% 2012 1 405131 87.47% 74.30% 2012 2 431420 85.41% 71.38% 2012 3 452507 83.44% 68.07% 2012 4 461118 80.62% 64.02%
As you can see the percentage of answered questions is a little worse than you thought, but the percentage of questions with a "good" answer is a little better. Personally I think the telling point here is not necessarily the number of questions but where the differences lie between my results and your own. For Q4 2008 you have 122,616 and I've got 39,557 questions, which implies that every question received over 3 answers. For Q2 2012 the difference is minimal, on average questions receiving about 1.2 answers.
I've also run this for questions with a score >= 0:
2008 3 17454 99.99% 99.84% 2008 4 38618 99.87% 98.13% 2009 1 53133 99.73% 96.24% 2009 2 74795 99.55% 94.26% 2009 3 97569 99.22% 92.52% 2009 4 111776 99.01% 91.77% 2010 1 141491 98.60% 90.82% 2010 2 157757 98.00% 88.54% 2010 3 185404 97.54% 87.34% 2010 4 203733 97.38% 86.91% 2011 1 265103 96.95% 85.88% 2011 2 293660 96.11% 84.35% 2011 3 305355 95.20% 82.55% 2011 4 308130 94.31% 81.54% 2012 1 392376 87.07% 74.14% 2012 2 417341 84.92% 71.18% 2012 3 436875 82.85% 67.81% 2012 4 443642 80.31% 64.00%
I think the surprise here is how little difference it makes. It reflects well on Stack Overflow that no matter if the question is not as good as it could be you are just as likely to get a "good" answer. Obviously, closed questions would skew this massively and as a number of "poor" questions get closed not too much can be read into this.
Lastly, here the same query is for questions with a score >= 1:
Year Quarter Questions Answered GoodAnswer ---- ------- --------- -------- ---------- 2008 3 15797 99.99% 99.88% 2008 4 32945 99.90% 98.87% 2009 1 41381 99.74% 98.18% 2009 2 53922 99.63% 97.38% 2009 3 65645 99.53% 96.71% 2009 4 69626 99.35% 95.97% 2010 1 99493 99.02% 94.66% 2010 2 104134 98.50% 93.04% 2010 3 116514 98.10% 92.16% 2010 4 122588 98.00% 91.82% 2011 1 152765 97.79% 91.31% 2011 2 169152 96.96% 89.90% 2011 3 167683 95.92% 88.18% 2011 4 165138 94.92% 86.80% 2012 1 183482 93.92% 85.34% 2012 2 180243 92.93% 83.66% 2012 3 173618 91.26% 80.88% 2012 4 188466 87.52% 74.33%
As you can see the number of answered questions and the number of questions answered "well" significantly improves though the same drop-off is observable.
My own conclusion from these statistics is that a finesse to the system to remove unanswered questions, or whatever it might be, is not what is required. The number of answered "good" questions at over 91% is, in my opinion, a pretty high number.
What seems to be needed is an increase in the number of people who answer questions. Whilst Stack Overflow has had an ever increasing number of people asking questions there hasn't been a commensurate increase in the number of people answering them.
I ran a little query to test this hypothesis:
Year Quarter Questioning Answering ---- ------- ----------- --------- 2008 3 6411 9007 2008 4 10724 13276 2009 1 13713 15995 2009 2 18929 21276 2009 3 24474 25631 2009 4 37388 36124 2010 1 47454 41411 2010 2 56895 46868 2010 3 66340 52398 2010 4 73162 59578 2011 1 95347 74931 2011 2 108828 78798 2011 3 118150 85501 2011 4 117156 89839 2012 1 151337 107792 2012 2 167394 116139 2012 3 182767 125379 2012 4 201461 133558
As you can see in the "early" days the number of users answering questions was more than the number of people asking them. This has now been completely reversed and the questioners are in the ascendant.
What the solution is, I'm not entirely sure. What seems certain though is that Stack Overflow needs to find a way of converting question askers into question answerers. Without flooding the place with crap answers.
