Quote from ammo:
the open interest,o.i. in the spy can be used by vol to figure out the monthly premium erosion ,where the take the market down and fatten up the puts,sell them,then take it up to fatten the calls ,sell them ,then bring it back somewhere in the middle to collect the premium erosion to zero at exp,as the exp gets nearer the oi sometimes changes so his 113-120 would change as we get nearer to exp