Essays on Instrumental Variables

This dissertation addresses issues that arise in the classic linear instrumental variables (IV) model when some of the underlying assumptions are violated.Economic

Robust Standard Errors in Small Samples: Some Practical Advice

We study the properties of heteroskedasticity-robust confidence intervals for regression parameters. We show that confidence intervals based on a degrees-of-freedom correction suggested by Bell and McCaffrey (2002) are a natural extension of a principled approach to the Behrens-Fisher problem. We suggest a further improvement for the case with clustering. We show that these standard errors can lead to substantial improvements in coverage rates even for samples with fifty or more clusters.We recommend that researchers routinely calculate the Bell-McCaffrey degrees-of-freedom adjustment to assess potential problems with conventional robust standard errors

Replication Data for: 'Shift-Share Designs: Theory and Inference'

The data and programs replicate tables and figures from "Shift-Share Designs: Theory and Inference", by Adao, Kolesar, and Morales. Please see the Roadmap files for additional details

Clustering, Spatial Correlations, and Randomization Inference

It is a standard practice in regression analyses to allow for clustering in the error covariance matrix if the explanatory variable of interest varies at a more aggregate level (e.g., the state level) than the units of observation (e.g., individuals). Often, however, the structure of the error covariance matrix is more complex, with correlations not vanishing for units in different clusters. Here, we explore the implications of such correlations for the actual and estimated precision of least squares estimators. Our main theoretical result is that with equal-sized clusters, if the covariate of interest is randomly assigned at the cluster level, only accounting for nonzero covariances at the cluster level, and ignoring correlations between clusters as well as differences in within-cluster correlations, leads to valid confidence intervals. However, in the absence of random assignment of the covariates, ignoring general correlation structures may lead to biases in standard errors. We illustrate our findings using the 5% public-use census data. Based on these results, we recommend that researchers, as a matter of routine, explore the extent of spatial correlations in explanatory variables beyond state-level clustering

Preventive and remedial conservation methodology for the collection specimens falling within palaeontology and mineralogy endangered by sulphide degradation products

The methodology is a complex system of procedures involving experimentally verified as well as modified already known methods, completely novel procedures, and their combinations. Many technologies as well as materials are employed for the first time in this field. Modifications of the existing methods bring either effectivity improvements, invasivity reduction or both. This methodology is the very first to cover all phases of the collection material treatment from acquisition to final storage. The methodology structuring follows logical succession of procedures. The theoretical chapter summarizes grounds of the degradation process, materials endangered with sulphide degradation and products of it. Area maps of types of sulphides labelled by process of origin, and a map of high-risk Czech localities are also involved. This introductory chapter is followed by part devoted to methods of field work and early care, suitable preventative collection care, and procedures of the remedial care. A special remedial care chapter for paper-documentation is also involved. This chapter is followed by instrumental methods useful for detection of degradation products and their influence on specimen condition. A separate part focuses on suitable ways of documentation. The safety-hazards chapter closes the methodology. The methodology is equipped with a list of references and flow diagrams helping the user to find the suitable procedure fast and easily. The methodology project was supported by the Ministry of Culture of the Czech Republic under NAKI DF12P01OVV031

Difference in Serum Endostatin Levels in Diabetic Patients with Critical Limb Ischemia Treated by Autologous Cell Therapy or Percutaneous Transluminal Angioplasty

The aim of this study was to compare the serum levels of the anti-angiogenic factor endostatin (S-endostatin) as a potential marker of vasculogenesis after autologous cell therapy (ACT) versus percutaneous transluminal angioplasty (PTA) in diabetic patients with critical limb ischemia (CLI). A total of 25 diabetic patients with CLI treated in our foot clinic during the period 2008–2014 with ACT generating potential vasculogenesis were consecutively included in the study; 14 diabetic patients with CLI who underwent PTA during the same period were included in a control group in which no vasculogenesis had occurred. S-endostatin was measured before revascularization and at 1, 3, and 6 months after the procedure. The effect of ACT and PTA on tissue ischemia was confirmed by transcutaneous oxygen pressure (TcPO 2 ) measurement at the same intervals. While S-endostatin levels increased significantly at 1 and 3 months after ACT (both P < 0.001), no significant change of S-endostatin after PTA was observed. Elevation of S-endostatin levels significantly correlated with an increase in TcPO 2 at 1 month after ACT ( r = 0.557; P < 0.001). Our study showed that endostatin might be a potential marker of vasculogenesis because of its significant increase after ACT in diabetic patients with CLI in contrast to those undergoing PTA. This increase may be a sign of a protective feedback mechanism of this anti-angiogenic factor

NBER WORKING PAPER SERIES MEASURING THE IMPACTS OF TEACHERS II: TEACHER VALUE-ADDED AND STUDENT OUTCOMES IN ADULTHOOD

John Friedman is currently on leave from Harvard, working at the National Economic Council. This work was completed before he began that position and does not represent the views of the NEC. W