How to Treat the Use of Grey Literature in Software Engineering

Context: Following on other scientific disciplines, such as health sciences, the use of grey literature (GL) is becoming widespread in Software Engineering (SE) research. Whilst the number of papers incorporating GL on SE is increasing, there is little empirically known about different aspects of the use of GL in SE research. In particular, there is a lack of excellent evaluation standard for the quality of GL. Aim: Our research is aimed at systematically reviewing the use of GL in SE, empirically exploring SE researchers' views on GL and providing a guide for using GL in SE and for quality assessment of the GL to be included. Method: We used a mixedmethods approach for this research. We carried out a Systematic Literature Review (SLR) of the use of GL in SE. Then we surveyed the authors of the papers included in the SLR (as GL users) and the invited experts in the SE community on the use of GL in SE research. Results: We systematically selected and reviewed 102 SE secondary studies that incorporate GL in SE research, from which we identified two groups based on their reporting: 1) 76 reviews only claim their use of GL; 2) 26 reviews report the results by including GL. We also obtained 20 replies from the GL users and 24 replies from the invited SE experts. Conclusion: There is no common understanding of the meaning of GL in SE. Researchers define the scopes and the definitions of GL in a variety of ways. We found five main reasons of using GL in SE research. The findings have enabled us to propose a conceptual model for how GL works in SE research lifecycle. In the next workThere is a need for research to develop guidelines for using GL in SE and for assessing quality of GL.