1 INTRODUCTION  
2 RESEARCH METHOD  
2.1 Planning the Systematic Mapping of the Literature  
2.2 Conducting the Systematic Mapping of the Literature  
2.3 Extracting and Analyzing Data  
2.4 Characterizing the Studies  
3 RESULTS  
4 DISCUSSION  
5 RELATED STUDIES  
6 THREATS TO VALIDITY  
7 CONCLUSION