1 INTRODUCTION  
2 RELATED WORK  
3 METHODOLOGY  
3.1 Categorization of User-Study Objectives  
3.2 Foundations of User Studies  
3.3 Impact of User Studies  
4 COMPREHENSIVE USER STUDY ANALYSIS  
4.1 Models and Explanations  
4.2 Measurements  
4.3 Experimental Design and Analysis  
5 FINDINGS OF USER STUDIES  
6 A GUIDELINE FOR XAI USER STUDY DESIGN  
7 FUTURE RESEARCH DIRECTIONS  
7.1 Towards Increasingly User-Centered XAI  
7.2 Open Research Problems  
8 CONCLUSION