The Credibility Revolution in Empirical Economics: How Better Research Design Is Taking the Con out of Econometrics

This essay reviews progress in empirical economics since Leamer's (1983) critique. Leamer highlighted the benefits of sensitivity analysis, a procedure in which researchers show how their results change with changes in specification or functional form. Sensitivity analysis has had a salutary but not a revolutionary effect on econometric practice. As we see it, the credibility revolution in empirical work can be traced to the rise of a design-based approach that emphasizes the identification of causal effects. Design-based studies typically feature either real or natural experiments and are distinguished by their prima facie credibility and by the attention investigators devote to making the case for a causal interpretation of the findings their designs generate. Design-based studies are most often found in the microeconomic fields of Development, Education, Environment, Labor, Health, and Public Finance, but are still rare in Industrial Organization and Macroeconomics. We explain why IO and Macro would do well to embrace a design-based approach. Finally, we respond to the charge that the design-based revolution has overreached.research design, natural experiments, structural models, quasi-experiments

Undergraduate econometrics instruction: through our classes, darkly

The past half-century has seen economic research become increasingly empirical, while the nature of empirical economic research has also changed. In the 1960s and 1970s, an empirical economist's typical mission was to "explain" economic variables like wages or GDP growth. Applied econometrics has since evolved to prioritize the estimation of specific causal effects and empirical policy analysis over general models of outcome determination. Yet econometric instruction remains mostly abstract, focusing on the search for "true models" and technical concerns associated with classical regression assumptions. Questions of research design and causality still take a back seat in the classroom, in spite of having risen to the top of the modern empirical agenda. This essay traces the divergent development of econometric teaching and empirical practice, arguing for a pedagogical paradigm shift

The Credibility Revolution in Empirical Economics: How Better Research Design is Taking the Con out of Econometrics

This essay reviews progress in empirical economics since Leamer’s (1983) critique. Leamer highlighted the benefits of sensitivity analysis, a procedure in which researchers show how their results change with changes in specification or functional form. Sensitivity analysis has had a salutary but not a revolutionary effect on econometric practice. As we see it, the credibility revolution in empirical work can be traced to the rise of a design-based approach that emphasizes the identification of causal effects. Design-based studies typically feature either real or natural experiments and are distinguished by their prima facie credibility and by the attention investigators devote to making the case for a causal interpretation of the findings their designs generate. Design-based studies are most often found in the microeconomic fields of Development, Education, Environment, Labor, Health, and Public Finance, but are still rare in Industrial Organization and Macroeconomics. We explain why IO and Macro would do well to embrace a design-based approach. Finally, we respond to the charge that the design-based revolution has overreached.

Measuring monopole and dipole polarizability of acoustic meta-atoms

We present a method to extract monopole and dipole polarizability from experimental measurements of two-dimensional acoustic meta-atoms. In contrast to extraction from numerical results, this enables all second-order effects and uncertainties in material properties to be accounted for. We apply the technique to 3D-printed labyrinthine meta-atoms of a variety of geometries. We show that the polarizability of structures with shorter acoustic path length agrees well with numerical results. However, those with longer path lengths suffer strong additional damping, which we attribute to the strong viscous and thermal losses in narrow channels

Off-Design Performance Analysis of a Solid-Oxide Fuel Cell/Gas Turbine Hybrid for Auxiliary Aerospace Power

A solid-oxide fuel cell/gas turbine hybrid system for auxiliary aerospace power is analyzed using 0-D and 1-D system-level models. The system is designed to produce 440 kW of net electrical power, sized for a typical long-range 300-passenger civil airplane, at both sea level and cruise flight level (12,500 m). In addition, a part power level of 250 kW is analyzed at the cruise condition, a requirement of the operating power profile. The challenge of creating a balanced system for the three distinct conditions is presented, along with the compromises necessary for each case. A parametric analysis is described for the cruise part power operating point, in which the system efficiency is maximized by varying the air flow rate. The system is compared to an earlier version that was designed solely for cruise operation. The results show that it is necessary to size the turbomachinery, fuel cell, and heat exchangers at sea level full power rather than cruise full power. The resulting estimated mass of the system is 1912 kg, which is significantly higher than the original cruise design point mass, 1396 kg. The net thermal efficiencies with respect to the fuel LHV are calculated to be 42.4 percent at sea level full power, 72.6 percent at cruise full power, and 72.8 percent at cruise part power. The cruise conditions take advantage of pre-compressed air from the on-board Environmental Control System, which accounts for a portion of the unusually high thermal efficiency at those conditions. These results show that it is necessary to include several operating points in the overall assessment of an aircraft power system due to the variations throughout the operating profile

Thermal modelling of Advanced LIGO test masses

High-reflectivity fused silica mirrors are at the epicentre of current advanced gravitational wave detectors. In these detectors, the mirrors interact with high power laser beams. As a result of finite absorption in the high reflectivity coatings the mirrors suffer from a variety of thermal effects that impact on the detectors performance. We propose a model of the Advanced LIGO mirrors that introduces an empirical term to account for the radiative heat transfer between the mirror and its surroundings. The mechanical mode frequency is used as a probe for the overall temperature of the mirror. The thermal transient after power build-up in the optical cavities is used to refine and test the model. The model provides a coating absorption estimate of 1.5 to 2.0 ppm and estimates that 0.3 to 1.3 ppm of the circulating light is scattered on to the ring heater.Comment: 14 pages, 9 figure

Development of Parametric Mass and Volume Models for an Aerospace SOFC/Gas Turbine Hybrid System

In aerospace power systems, mass and volume are key considerations to produce a viable design. The utilization of fuel cells is being studied for a commercial aircraft electrical power unit. Based on preliminary analyses, a SOFC/gas turbine system may be a potential solution. This paper describes the parametric mass and volume models that are used to assess an aerospace hybrid system design. The design tool utilizes input from the thermodynamic system model and produces component sizing, performance, and mass estimates. The software is designed such that the thermodynamic model is linked to the mass and volume model to provide immediate feedback during the design process. It allows for automating an optimization process that accounts for mass and volume in its figure of merit. Each component in the system is modeled with a combination of theoretical and empirical approaches. A description of the assumptions and design analyses is presented