  Stack Overflow is the most popular CQA for programmers on the web with 2.05M
users, 5.1M questions and 9.4M answers. Stack Overflow has explicit, detailed
guidelines on how to post questions and an ebullient moderation community.
Despite these precise communications and safeguards, questions posted on Stack
Overflow can be extremely off topic or very poor in quality. Such questions can
be deleted from Stack Overflow at the discretion of experienced community
members and moderators. We present the first study of deleted questions on
Stack Overflow. We divide our study into two parts (i) Characterization of
deleted questions over approx. 5 years (2008-2013) of data, (ii) Prediction of
deletion at the time of question creation. Our characterization study reveals
multiple insights on question deletion phenomena. We observe a significant
increase in the number of deleted questions over time. We find that it takes
substantial time to vote a question to be deleted but once voted, the community
takes swift action. We also see that question authors delete their questions to
salvage reputation points. We notice some instances of accidental deletion of
good quality questions but such questions are voted back to be undeleted
quickly. We discover a pyramidal structure of question quality on Stack
Overflow and find that deleted questions lie at the bottom (lowest quality) of
the pyramid. We also build a predictive model to detect the deletion of
question at the creation time. We experiment with 47 features based on User
Profile, Community Generated, Question Content and Syntactic style and report
an accuracy of 66%. Our feature analysis reveals that all four categories of
features are important for the prediction task. Our findings reveal important
suggestions for content quality maintenance on community based question
answering websites.
