party ▁ad ▁servers ▁that ▁dist ribute ▁ad s ▁on ▁the ▁website ( s ) ▁content ▁pages . ▁These ▁tags ▁are ▁placed ▁on ▁a ▁web ▁page ▁and ▁when ▁rendered , ▁employ ing ▁a ▁" Cor rel ator " ▁( a ▁linear ▁correlation ▁control .) ▁The ▁ad ▁space ▁is ▁then ▁" mark ed ▁up ," ▁an ▁" ad ▁request ” ▁( server ▁log ▁impression ) ▁is ▁recorded , ▁and ▁the ▁Cor rel ator ▁begins ▁communic ating ▁with ▁the ▁web ▁page , ▁browser ▁and ▁ad ▁unit ▁( ad ▁space ) ▁embedded ▁in ▁the ▁webpage ▁content . ▁The ▁Cor rel ator ▁can ▁collect ▁additional ▁non - private ▁information ▁from ▁the ▁vie wer ’ s ▁browser , ▁including ▁the ▁vie wer ’ s ▁operating ▁system , ▁browser ▁type ▁and ▁version ▁and ▁a ▁list ▁of ▁other ▁ad s ▁that ▁were ▁previously ▁rendered ▁on ▁the ▁page ▁to ▁prevent ▁dup lication ▁of ▁ad s ▁on ▁the ▁content ▁page . ▁Once ▁any ▁portion ▁of ▁the ▁ad ▁unit ▁( defin able ), ▁on ▁a ▁vie wer ' s ▁in ▁focus ▁web ▁page , ▁hits ▁the ▁visible ▁area ▁of ▁the ▁browser ▁window ▁a ▁request ▁is ▁sent ▁to ▁an ▁ad ▁content ▁server ▁to ▁deliver ▁an ▁advert is ement . ▁ ▁Once ▁the ▁ad ▁content ▁is ▁loaded ▁and ▁rendered ▁an ▁" Ad ▁Render ed " ▁is ▁reported . ▁The ▁Cor rel ator ▁continues ▁to ▁monitor ▁the ▁ad ▁space ▁for ▁each ▁individual ▁ad ▁on ▁the ▁web ▁page ▁and ▁its ▁relation ▁to ▁the ▁browser ▁window ▁dimensions , ▁scrolling ▁position ▁and ▁web ▁page ▁focus , ▁considering ▁if ▁the ▁vie