for normal TCH drop
TN_DROP = TFNDROP + TFNDROPSUB + THNDROP + THNDROPSUB
and formula for subscriber perceived drop on TCH is T_DR_S=(TN_DROP/N_CALLs)*100
TFNDROP:- The number of dropped full-rate TCH in underlaid subcell.
TFNDROPSUB:- the number for the drop for hlaf rate TCH.
THNDROP and THNdropsun is total number for drop
hope it help you but dear this definition is not obliged to put you can modified as the operator want for example in other operator for call drop the took in to consideration the drop which happen due to handover which makeit call drop less than that with out handover this is way you can say tricky way .