Stocks slipped as the latest round of earnings deflated investors. Express Scripts, the pharmacy benefits manager, fell after fourth-quarter results slipped, and Groupon plunged as it expects to post a loss. Markets have gained this month on optimism the economy will start to pull out of its winter slump as the weather improves. Investors are ‘‘giving the economic data points a bit of a free pass, but at the same time they’re not fully convinced either,’’ said one chief market strategist. Ameren, a utility based in St. Louis, was among the Friday’s winners. Ameren’s earnings beat expectations after cold weather boosted demand for gas and electricity.