Brier Score and Brier Skill Score is defined as the mean square probability and could be expressed most simply as: BS(f) = 1-(1/n sum((f-o)**2)) and BSS=(BS(f)-BS(c))/(1-BS(c)); 0 of BSS means no skill compare to climatology. 0-1.0 means forecats has more skill than climatology, -1.0-0 means other way.