  We explore the dynamics of user performance in collaborative knowledge
production by studying the quality of answers to questions posted on Stack
Exchange. We propose four indicators of answer quality: answer length, the
number of code lines and hyperlinks to external web content it contains, and
whether it is accepted by the asker as the most helpful answer to the question.
Analyzing millions of answers posted over the period from 2008 to 2014, we
uncover regular short-term and long-term changes in quality. In the short-term,
quality deteriorates over the course of a single session, with each successive
answer becoming shorter, with fewer code lines and links, and less likely to be
accepted. In contrast, performance improves over the long-term, with more
experienced users producing higher quality answers. These trends are not a
consequence of data heterogeneity, but rather have a behavioral origin. Our
findings highlight the complex interplay between short-term deterioration in
performance, potentially due to mental fatigue or attention depletion, and
long-term performance improvement due to learning and skill acquisition, and
its impact on the quality of user-generated content.
