The Survey of Income and Program Participation (SIPP) is a longitudinal survey that measures the short-term dynamics of key socioeconomic characteristics including poverty status. However, since its inception, estimated monthly poverty rates from the end of one panel and the beginning of another are inconsistent and poverty rates from the first wave are higher than those from the second wave. (See Huggins and Winters and Czajka, Mabli, and Cody for previous work.) This study will address anomalies in the 2004 panel by decomposing changes in monthly poverty rates into cross-sectional and within person differences. To explain the relationship between sample composition and poverty, we will examine changes in characteristics included in and exogenous to weighting. To analyze within person changes in poverty, we will examine income changes.