There has been a general decline in the dropout rate and an increase in the high school completion rate over the last three decades. This research investigates causes for the decline in the dropout rate over the periods using decomposition analysis. Traditional cross-section analysis was inadequate to perform this task. Using the two cohort surveys of the National Longitudinal Survey of Youth (NLSY) in the 1980s and 2000s, we separated changes in characteristics into two parts: explained change and unexplained change. Results of the research suggest that the common explanations for the characteristic of school dropout account for little of the decline of the rate. Relatively unnoticeable factors such as location and regions contributed to the decline of the dropout rate while socioeconomic, personal, familial factors contributed to increase the dropout rate. (Contains 3 tables.)