Since the transition of the research focus from longitudual to perpendicular recording system in early 2000, there has been significant progress in understanding and developing metrologies to characterize the switching field distribution (SFD). However, most of the metrologies developed for characterizing SFD have relied on mean field assumptions. In this paper, we investigate the mean field assumptions commonly used in the SFD metrologies. For micromagnetic simulation data without intergranular exchange interactions, we find that the interaction fields are independent of the local environment of the spins, which suggests that the mean field approximation is applicable. Moreover, the dependency between the interaction field and the magnetization is rather linear with and without thermal fluctuations. For the experimental data obtained from a sample with moderate intergranular exchange interactions, we find that the mean field approximation is reasonable in situations with thermal fluctuation. But for the case without thermal fluctuation, such approximation might be called into question.