  Background: Software development results in the production of various types
of artifacts: source code, version control system metadata, bug reports,
mailing list conversations, test data, etc. Empirical software engineering
(ESE) has thrived mining those artifacts to uncover the inner workings of
software development and improve its practices. But which artifacts are studied
in the field is a moving target, which we study empirically in this paper.Aims:
We quantitatively characterize the most frequently mined and co-mined software
artifacts in ESE research and the research purposes they support.Method: We
conduct a meta-analysis of artifact mining studies published in 11 top
conferences in ESE, for a total of 9621 papers. We use natural language
processing (NLP) techniques to characterize the types of software artifacts
that are most often mined and their evolution over a 16-year period
(2004-2020). We analyze the combinations of artifact types that are most often
mined together, as well as the relationship between study purposes and mined
artifacts.Results: We find that: (1) mining happens in the vast majority of
analyzed papers, (2) source code and test data are the most mined artifacts,
(3) there is an increasing interest in mining novel artifacts, together with
source code, (4) researchers are most interested in the evaluation of software
systems and use all possible empirical signals to support that goal.
