CDO

Equitable threat score (Gilbert skill score)-Equation for equitable threat score (also denoted GSS)
where Equation for hits due to random chance

http://www.eumetcal.org/resources/ukmeteocal/verification/www/english/msg/ver_categ_forec/uos2/uos2_ko4.htm

I would like to use the score to evaluate WRF simulated precipitation against observations, but I don't know how to calculate it.
Any pointers would be greatly appreciated!