Variance reduction ratio (VRR) is generally considered as an important index in characterizing the continuous powder mixing. Although the capacity of the mixer to smooth out feeder fluctuations can be expressed by the VRR, few studies are performed quantitatively in this area. The feeder effects are investigated on the solid mixer through the Fourier series analysis. The VRR is deduced quantitatively using the Fourier series of the feed rate variability and the residence time distribution (RTD), which facilitate the explicit decomposition of VRR into intensities of different frequency components. It provides a novel model to determine whether the integrated feeder-mixer system satisfies specific solid mixing performance criteria, and provides guidelines of system improvement. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1144-1153, 2011