Bootstrap-Based Improvements for Inference with Clustered Errors

Microeconometrics researchers have increasingly realized the essential need to account for any within-group dependence in estimating standard errors of regression parameter estimates. The typical preferred solution is to calculate cluster-robust or sandwich standard errors that permit quite general heteroskedasticity and within-cluster error correlation, but presume that the number of clusters is large. In applications with few (5-30) clusters, standard asymptotic tests can over-reject considerably. We investigate more accurate inference using cluster bootstrap-t procedures that provide asymptotic refinement. These procedures are evaluated using Monte Carlos, including the much-cited differences-in-differences example of Bertrand, Mullainathan and Duflo (2004). In situations where standard methods lead to rejection rates in excess of ten percent (or more) for tests of nominal size 0.05, our methods can reduce this to five percent. In principle a pairs cluster bootstrap should work well, but in practice a Wild cluster bootstrap performs better.clustered errors; random effects; cluster robust; sandwich; bootstrap; bootstrap-t; clustered bootstrap; pairs bootstrap; wild bootstrap.

Bootstrap-Based Improvements for Inference with Clustered Errors

Researchers have increasingly realized the need to account for within-group dependence in estimating standard errors of regression parameter estimates. The usual solution is to calculate cluster-robust standard errors that permit heteroskedasticity and within-cluster error correlation, but presume that the number of clusters is large. Standard asymptotic tests can over-reject, however, with few (5-30) clusters. We investigate inference using cluster bootstrap-t procedures that provide asymptotic refinement. These procedures are evaluated using Monte Carlos, including the example of Bertrand, Duflo and Mullainathan (2004). Rejection rates of ten percent using standard methods can be reduced to the nominal size of five percent using our methods.

Robust Inference with Multi-way Clustering

In this paper we propose a new variance estimator for OLS as well as for nonlinear estimators such as logit, probit and GMM, that provcides cluster-robust inference when there is two-way or multi-way clustering that is non-nested. The variance estimator extends the standard cluster-robust variance estimator or sandwich estimator for one-way clustering (e.g. Liang and Zeger (1986), Arellano (1987)) and relies on similar relatively weak distributional assumptions. Our method is easily implemented in statistical packages, such as Stata and SAS, that already offer cluster-robust standard errors when there is one-way clustering. The method is demonstrated by a Monte Carlo analysis for a two-way random effects model; a Monte Carlo analysis of a placebo law that extends the state-year effects example of Bertrand et al. (2004) to two dimensions; and by application to two studies in the empirical public/labor literature where two-way clustering is present.

"Quantum Geometric Nesting'' and Solvable Model Flat-Band Systems

We introduce the concept of "quantum geometric nesting'' (QGN) to characterize the idealized ordering tendencies of certain flat-band systems implicit in the geometric structure of the flat-band subspace. Perfect QGN implies the existence of an infinite class of local interactions that can be explicitly constructed and give rise to solvable ground states with various forms of possible fermion bi-linear order, including flavor ferromagnetism, density waves, and superconductivity. For the ideal Hamiltonians constructed in this way, we show that certain aspects of the low-energy spectrum can also be exactly computed including, in the superconducting case, the phase stiffness. Examples of perfect QGN include flat bands with certain symmetries (e.g. chiral or time-reversal), and non-symmetry-related cases exemplified with an engineered model for pair-density-wave. Extending this approach, we obtain exact superconducting ground states with nontrivial pairing symmetry

A Missing Piece of the Puzzle: Patient and Provider Perspectives on Pain Management Needs and Opioid Prescribing in Inflammatory Bowel Disease Care

BACKGROUND: Recent data have shown high rates of opioid misuse among inflammatory bowel disease (IBD) patients. We conducted a qualitative study to explore IBD patient and provider perceptions and experiences with pain management and opioid prescribing. METHODS: We conducted a focus group with IBD patients and semistructured interviews with IBD-focused physicians and nurses. We used an inductive approach for analysis and the constant comparison method to develop and refine codes and identify prominent themes. We analyzed interview and focus group data concurrently to triangulate themes. RESULTS: Nine patients and 10 providers participated. We grouped themes into 3 categories: (1) current practices to manage pain; (2) perceived pain management challenges; and (3) suggestions to optimize pain management. In the first category (current practices), both patients and providers reported building long-term patient-provider relationships and the importance of exploring nonpharmacologic pain management strategies. Patients reported proactively trying remedies infrequently recommended by IBD providers. In the second category (pain management challenges), patients and providers reported concerns about opioid use and having limited options to treat pain safely. Patients discussed chronic pain and having few solutions to manage it. In the third category, providers shared suggestions for improvement such as increasing use of nonpharmacologic pain management strategies and enhancing care coordination. CONCLUSIONS: Despite some common themes between the 2 groups, we identified some pain management needs (eg, addressing chronic pain) that matter to patients but were seldom discussed by IBD providers. Addressing these areas of potential disconnect is essential to optimize pain management safety in IBD care

A task and performance analysis of endoscopic submucosal dissection (ESD) surgery

BACKGROUND: ESD is an endoscopic technique for en bloc resection of gastrointestinal lesions. ESD is a widely-used in Japan and throughout Asia, but not as prevalent in Europe or the US. The procedure is technically challenging and has higher adverse events (bleeding, perforation) compared to endoscopic mucosal resection. Inadequate training platforms and lack of established training curricula have restricted its wide acceptance in the US. Thus, we aim to develop a Virtual Endoluminal Surgery Simulator (VESS) for objective ESD training and assessment. In this work, we performed task and performance analysis of ESD surgeries. METHODS: We performed a detailed colorectal ESD task analysis and identified the critical ESD steps for lesion identification, marking, injection, circumferential cutting, dissection, intraprocedural complication management, and post-procedure examination. We constructed a hierarchical task tree that elaborates the order of tasks in these steps. Furthermore, we developed quantitative ESD performance metrics. We measured task times and scores of 16 ESD surgeries performed by four different endoscopic surgeons. RESULTS: The average time of the marking, injection, and circumferential cutting phases are 203.4 (σ: 205.46), 83.5 (σ: 49.92), 908.4 s. (σ: 584.53), respectively. Cutting the submucosal layer takes most of the time of overall ESD procedure time with an average of 1394.7 s (σ: 908.43). We also performed correlation analysis (Pearson's test) among the performance scores of the tasks. There is a moderate positive correlation (R = 0.528, p = 0.0355) between marking scores and total scores, a strong positive correlation (R = 0.7879, p = 0.0003) between circumferential cutting and submucosal dissection and total scores. Similarly, we noted a strong positive correlation (R = 0.7095, p = 0.0021) between circumferential cutting and submucosal dissection and marking scores. CONCLUSIONS: We elaborated ESD tasks and developed quantitative performance metrics used in analysis of actual surgery performance. These ESD metrics will be used in future validation studies of our VESS simulator

No Forest Left Behind

Mitigating climate change by reducing deforestation should involve incentives for countries that currently have high forest cover and low deforestation rates

Pulse radiolysis of liquid water using picosecond electron pulses produced by a table-top terawatt laser system

A laser based electron generator is shown, for the first time, to produce sufficient charge to conduct time resolved investigations of radiation induced chemical events. Electron pulses generated by focussing terawatt laser pulses into a supersonic helium gas jet are used to ionize liquid water. The decay of the hydrated electrons produced by the ionizing electron pulses is monitored with 0.3 μs time resolution. Hydrated electron concentrations as high as 22 μM were generated. The results show that terawatt lasers offer both an alternative to linear accelerators and a means to achieve subpicosecond time resolution for pulse radiolysis studies. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69949/2/RSINAK-71-6-2305-1.pd

Learning From the Implementation of a Surgical Opioid Reduction Initiative in an Integrated Health System: A Qualitative Study Among Providers and Patients

BACKGROUND: Surgical opioid overprescribing can result in long-term use or misuse. Between July 2018 and March 2019, the multicomponent intervention, Minimizing Opioid Prescribing in Surgery (MOPiS) was implemented in the general surgery clinics of five hospitals and successfully reduced opioid prescribing. To date, various studies have shown a positive outcome of similar reduction initiatives. However, in addition to evaluating the impact on clinical outcomes, it is important to understand the implementation process of an intervention to extend sustainability of interventions and allow for dissemination of the intervention into other contexts. This study aims to evaluate the contextual factors impacting intervention implementation. METHODS: We conducted a qualitative study with semi-structured interviews held with providers and patients of the general surgery clinics of five hospitals of a single health system between March and November of 2019. Interview questions focused on how contextual factors affected implementation of the intervention. We coded interview transcripts deductively, using the Consolidated Framework for Implementation Research (CFIR) to identify the relevant contextual factors. Content analyses were conducted using a constant comparative approach to identify overarching themes. RESULTS: We interviewed 15 clinicians (e.g., surgeons, nurses), 1 quality representative, 1 scheduler, and 28 adult patients and identified 3 key themes. First, we found high variability in the responses of clinicians and patients to the intervention. There was a strong need for intervention components to be locally adaptable, particularly for the format and content of the patient and clinician education materials. Second, surgical pain management should be recognized as a team effort. We identified specific gaps in the engagement of team members, including nurses. We also found that the hierarchical relationships between surgical residents and attendings impacted implementation. Finally, we found that established patient and clinician views on opioid prescribing were an important facilitator to effective implementation. CONCLUSION: Successful implementation of a complex set of opioid reduction interventions in surgery requires locally adaptable elements of the intervention, a team-centric approach, and an understanding of patient and clinician views regarding changes being proposed