▁“ number ▁of ▁different ▁prospect s ▁reached ” ▁was ▁removed ▁from ▁the ▁equation , ▁and ▁a ▁new ▁C PM ▁ ▁equation ▁was ▁created ▁for ▁the ▁internet : ▁▁ 3 . ▁C PM ▁for ▁the ▁Internet : ▁ ▁However ▁the ▁assumption ▁that ▁an ▁ad ▁requested ▁from ▁an ▁ad ▁server ▁is ▁always ▁visible ▁when ▁the ▁vie wer ▁is ▁on ▁the ▁requested ▁page ▁was ▁wrong ▁because ▁of ▁a ▁few ▁technical ▁reasons ▁and ▁the ▁fact ▁that ▁the ▁web ▁page ▁is ▁usually ▁longer ▁than ▁the ▁height ▁of ▁a ▁computer ▁screen . ▁Event ually ▁it ▁became ▁apparent ▁that ▁a ▁large ▁number ▁of ▁ad ▁impress ions ▁measured ▁for ▁C PM ▁pr icing ▁actually ▁never ▁rendered ▁in ▁the ▁visible ▁area ▁of ▁a ▁vie wer ’ s ▁browser ▁screen . ▁ ▁C PM V ▁( Cost ▁per ▁Th ous and ▁View able ▁ad s ) ▁Until ▁ 2 0 1 0 ▁it ▁was ▁very ▁common ▁for ▁large ▁publish ers ▁to ▁charge ▁for ▁most ▁of ▁their ▁advert ising ▁invent ory ▁on ▁a ▁C PM ▁or ▁C PT ▁basis . ▁A ▁related ▁term , ▁effective ▁cost ▁per ▁m ille ▁( e CP M ), ▁is ▁used ▁to ▁measure ▁the ▁effect iveness ▁of ▁advert ising ▁invent ory ▁sold ▁( by ▁the ▁publish er ) ▁via ▁a ▁C PC , ▁C PA , ▁or ▁C PT ▁basis . ▁ ▁Part ial ly ▁to ▁avoid ▁the ▁limitations ▁of ▁server ▁side ▁impression ▁method ology ▁many ▁models ▁emer ged ▁that ▁were ▁based ▁on ▁direct ▁response : ▁ ▁C PC ▁- ▁Cost ▁per ▁click ▁Through ▁ ▁C PL ▁- ▁Cost