Internal Threats to Validity: This study has used only
three databases. Recently, there are some other open-access
paper archival repositories. Including them in the study may
introduce many other articles and, hence, reveal different
paper statistics. Moreover, there is invariably a time delay
for inclusion of papers into each database. For instance, the
proceedings of a conference held in December 2011 may not
have appeared in the three databases at the time of data
extraction. To address this threat, one may conduct several
rounds of data extraction until the dataset is stabilized, but
this will take an indefinite amount of time and a great deal
more effort to track the evolution.