Biased Encouragements and Heterogeneous Effects in an Instrumental Variable Study of Emergency General Surgical Outcomes

TL;DR

This study evaluates the effects of surgical versus non-surgical management for gastrointestinal conditions using instrumental variables, developing a new sensitivity analysis to account for effect heterogeneity and assessing robustness of findings.

Contribution

It introduces a novel sensitivity analysis method that handles arbitrary effect heterogeneity in IV studies, applied to surgical outcomes.

Findings

Non-surgical management reduces hospital stay and complications.

Results are more robust for non-septic patients despite smaller effects.

Sensitivity analysis shows non-septic results withstand hidden bias better.

Abstract

We investigate the efficacy of surgical versus non-surgical management for two gastrointestinal conditions, colitis and diverticulitis, using observational data. We deploy an instrumental variable design with surgeons' tendencies to operate as an instrument. Assuming instrument validity, we find that non-surgical alternatives can reduce both hospital length of stay and the risk of complications, with estimated effects larger for septic patients than for non-septic patients. The validity of our instrument is plausible but not ironclad, necessitating a sensitivity analysis. Existing sensitivity analyses for IV designs assume effect homogeneity, unlikely to hold here because of patient-specific physiology. We develop a new sensitivity analysis that accommodates arbitrary effect heterogeneity and exploits components explainable by observed features. We find that the results for non-septic patients prove more robust to hidden bias despite having smaller estimated effects. For non-septic patients, two individuals with identical observed characteristics would have to differ in their odds of assignment to a high tendency to operate surgeon by a factor of 2.34 to overturn our finding of a benefit for non-surgical management in reducing length of stay. For septic patients, this value is only 1.64. Simulations illustrate that this phenomenon may be explained by differences in within-group heterogeneity.