1. INTRODUCTION

2. OBSERVATIONS  
2.1. Far-infrared dust observations  
2.2. Atomic gas  
2.3. Molecular gas traced by CO

3. METHOD  
3.1. Benefits of the stacking analysis on all-sky surveys  
3.2. Milky Way Cirrus Subtraction  
3.3. Stacking of the dust emission  
3.4. Modeling of the dust emission  
3.5. Self-calibration of the dust emissivity

4. RESULTS  
4.1. Dust parameter variations with gas surface density  
4.2. Dust abundance vs gas surface density  
4.3. Comparison to elemental depletions

5. DISCUSSION  
5.1. $G/D$ as a ratio or a slope  
5.2. Why does the dust abundance vary with surface density?  
5.3. Possible effects of optically thick H i, CO-dark $H_{2}$, emissivity variations, and resolution  
5.4. Implications

6. CONCLUSION