Ever smaller critical dimensions (CD) and tighter budgets call for minimization of device-to-device differences in the fab. While all devices typically share the same design rule for the targeted feature on any given layer, pattern density variation became a significant contribution to the observed CD variation. In this study pattern density effect on STI (Shallow Trench Isolation) - type etch was characterized on local (hundreds of microns) and global (wafer) scales while keeping pitch the same and ARDE (Aspect Ratio Dependent Etch)-effects constant. One the challenges were to determine what constitutes the local scale and how data can be collected without using special masks. It was found that etch bias is twice more sensitive to variation in local pattern density than to the global. Methodology for proper accounting pattern density-induced CD variation in process development as well as strategies for compensating these effects will be discussed.