We develop an econometric framework to study merger efficiencies. A range of classification methods are employed to determine the sign of the merger's effect on market-specific output levels. Building on these classifications, and on familiar oligopoly theory results, we compute bounds on market-specific reductions in marginal costs. We apply this framework to the airline industry and study the 2013 merger of US Airways and American Airlines. The effect of the merger on output was positive in more than half of the markets where the sign of this effect could be determined, and in at least 44% of the total number of markets that were directly affected by the merger and where the market structure was otherwise stable. Pro-competitive effects are more prevalent in larger markets. Averaging across the markets experiencing output expansions, the lower bound on the marginal cost reduction is slightly above 2 USD, capturing 0.8% of the market price. The framework thus provides insights regarding the nature and magnitude of merger efficiencies. We stress that these are preliminary results.