Everyone has an opinion on pop-ups. Love ‘em for a discount, loathe ‘em when they gum up your experience. But, at their core, they deliver what you need to know most -- who is your shopper? -- in the form of an email address or sms number. Location proves to be a key variable for merchants to tinker with; while banners provide the strongest completion rate, the overall yield is best from center pop-ups.