ARCH-COMP21 Category Report: Stochastic Models. - Alessandro Abate; Henk A. P. Blom; Marc Bouissou; Nathalie Cauchi; Hassane Chraibi; Joanna Delicaris; Sofie Haesaert; Arnd Hartmanns; Mahmoud Khaled; Abolfazl Lavaei; Hao Ma; Kaushik Mallik; et al. (ARCH@ADHS, 2021)