This volume deals with advanced topics and specific problems in applied econometrics. Part III introduces advanced topics in econometric theory and contains three chapters. Chapter 7 deals with modeling issues and some phenomena that occur when the dataset you have collected has certain problems that need special attention for your results to be reliable. Chapter 8 analyzes the concepts and models that are not linear in their forms. Chapter 9 introduces several interesting models in advanced time-series techniques when a dataset is not stationary. Part IV applies the theoretical concepts learned in the previous chapters into empirical research. This part also consists of three chapters. Chapter 10 discusses the problem of selection bias and correcting methods. Chapter 11 introduces the regression discontinuity design and differences-in-differences models. Chapter 12 presents steps to carry out an empirical research project and provides strategies to avoid pitfalls in applied econometrics.