An article entitled "The Science of Polling" in Newsweek stated: "Ensuring a random sample also requires something beyond statistics. During Reagan's 1984 re-election campaign, his internal ‘tracking' polls showed him well ahead of Mondale - except on Friday nights. ‘They went into a panic every week until they figured it out,' says Frank Lutz, a political science professor at the University of Pennsylvania ... The explanation: registered Republicans, on average more flush than their Democratic counterparts, were more likely to go out on the town Friday nights and not be home to answer the pollster's phone call." What errors were the pollsters encountering in their sampling procedure on Fridays?