I. Introduction  
A. Related Work and Contributions  

II. Background  
A. Non-Negative Matrix Factorization (NMF)  
B. NMF for Topic Modeling  
C. Hierarchical NMF (HNMF)  

III. Population-Based HNMF (PHNMF)  

IV. Experimental Setup  
B. Real Data  
C. Experimental Setup  

V. Results  
A. Synthetic Data Results  
B. Real Survey Data Results