Melody Y. Huang

+ Research

Pre-Prints

Minimax Regret Estimation for Generalizing Heterogeneous Treatment Effects with Multisite Data
with Yi Zhang and Kosuke Imai

ArXiv

To test scientific theories and develop individualized treatment rules, researchers often wish to learn heterogeneous treatment effects that can be consistently found across diverse populations and contexts. We consider the problem of generalizing heterogeneous treatment effects (HTE) based on data from multiple sites. A key challenge is that a target population may differ from the source sites in unknown and unobservable ways. This means that the estimates from site-specific models lack external validity, and a simple pooled analysis risks bias. We develop a robust CATE (conditional average treatment effect) estimation methodology with multisite data from heterogeneous populations. We propose a minimax-regret framework that learns a generalizable CATE model by minimizing the worst-case regret over a class of target populations whose CATE can be represented as convex combinations of site-specific CATEs. Using robust optimization, the proposed methodology accounts for distribution shifts in both individual covariates and treatment effect heterogeneity across sites. We show that the resulting CATE model has an interpretable closed-form solution, expressed as a weighted average of site-specific CATE models. Thus, researchers can utilize a flexible CATE estimation method within each site and aggregate site-specific estimates to produce the final model. Through simulations and a real-world application, we show that the proposed methodology improves the robustness and generalizability of existing approaches.

Does AI help humans make better decisions? A statistical evaluation framework for experimental and observational studies
with Eli Ben-Michael, D. James Greiner, Kosuke Imai, Zhichao Jiang, and Sooahn Shin

ArXiv

The use of Artificial Intelligence (AI), or more generally data-driven algorithms, has become ubiquitous in today's society. Yet, in many cases and especially when stakes are high, humans still make final decisions. The critical question, therefore, is whether AI helps humans make better decisions compared to a human-alone or AI-alone system. We introduce a new methodological framework to empirically answer this question with a minimal set of assumptions. We measure a decision maker's ability to make correct decisions using standard classification metrics based on the baseline potential outcome. We consider a single-blinded and unconfounded treatment assignment, where the provision of AI-generated recommendations is assumed to be randomized across cases with humans making final decisions. Under this study design, we show how to compare the performance of three alternative decision-making systems--human-alone, human-with-AI, and AI-alone. Importantly, the AI-alone system includes any individualized treatment assignment, including those that are not used in the original study. We also show when AI recommendations should be provided to a human-decision maker, and when one should follow such recommendations. We apply the proposed methodology to our own randomized controlled trial evaluating a pretrial risk assessment instrument. We find that the risk assessment recommendations do not improve the classification accuracy of a judge's decision to impose cash bail. Furthermore, we find that replacing a human judge with algorithms--the risk assessment score and a large language model in particular--leads to a worse classification performance.

Design sensitivity and its implication on weighted observational studies
with Dan Soriano and Sam Pimentel

ArXiv

Sensitivity to unmeasured confounding is not typically a primary consideration in designing treated-control comparisons in observational studies. We introduce a framework allowing researchers to optimize robustness to omitted variable bias at the design stage using a measure called design sensitivity. Design sensitivity, which describes the asymptotic power of a sensitivity analysis, allows transparent assessment of the impact of different estimation strategies on sensitivity. We apply this general framework to two commonly-used sensitivity models, the marginal sensitivity model and the variance-based sensitivity model. By comparing design sensitivities, we interrogate how key features of weighted designs, including choices about trimming of weights and model augmentation, impact robustness to unmeasured confounding, and how these impacts may differ for the two different sensitivity models. We illustrate the proposed framework on a study examining drivers of support for the 2016 Colombian peace agreement.

Publications

Overlap violations in external validity
Annals of Applied Statistics (2024+)

ArXiv

Estimating externally valid causal effects is a foundational problem in the social and biomedical sciences. Generalizing or transporting causal estimates from an experimental sample to a target population of interest relies on an overlap assumption between the experimental sample and the target population--i.e., all units in the target population must have a non-zero probability of being included in the experiment. In practice, having full overlap between an experimental sample and a target population can be implausible. In the following paper, we introduce a framework for considering external validity in the presence of overlap violations. We propose a novel bias decomposition that parameterizes the bias from an overlap violation into two components: (1) the proportion of units omitted, and (2) the degree to which omitting the units moderates the treatment effect. The bias decomposition offers an intuitive and straightforward approach to conducting sensitivity analysis to assess robustness to overlap violations. Furthermore, we introduce a suite of sensitivity tools in the form of summary measures and benchmarking, which help researchers consider the plausibility of the overlap violations. We apply the proposed framework on an experiment evaluating the impact of a cash transfer program in Northern Uganda.

Variance-based sensitivity analysis for weighted estimators result in more informative bounds
with Sam Pimentel
Biometrika (2024+)

Article

Weighting methods are popular tools for estimating causal effects; assessing their robustness under unobserved confounding is important in practice. In the following paper, we introduce a new set of sensitivity models called "variance-based sensitivity models." Variance-based sensitivity models characterize the bias from omitting a confounder by bounding the distributional differences that arise in the weights from omitting a confounder, with several notable innovations over existing approaches. First, the variance-based sensitivity models can be parameterized with respect to a simple R^2 parameter that is both standardized and bounded. We introduce a formal benchmarking procedure that allows researchers to use observed covariates to reason about plausible parameter values in an interpretable and transparent way. Second, we show that researchers can estimate valid confidence intervals under a set of variance-based sensitivity models, and provide extensions for researchers to incorporate their substantive knowledge about the confounder to help tighten the intervals. Last, we highlight the connection between our proposed approach and existing sensitivity analyses, and demonstrate both, empirically and theoretically, that variance-based sensitivity models can provide improvements on both the stability and tightness of the estimated confidence intervals over existing methods. We illustrate our proposed approach on a study examining blood mercury levels using the National Health and Nutrition Examination Survey (NHANES).

Towards generalizing inferences from trials to target populations
with Harsh Parikh
Harvard Data Science Review (2024+)

ArXiv

Randomized Controlled Trials (RCTs) are pivotal in generating internally valid estimates with minimal assumptions, serving as a cornerstone for researchers dedicated to advancing causal inference methods. However, extending these findings beyond the experimental cohort to achieve externally valid estimates is crucial for broader scientific inquiry. This paper delves into the forefront of addressing these external validity challenges, encapsulating the essence of a multidisciplinary workshop held at the Institute for Computational and Experimental Research in Mathematics (ICERM), Brown University, in Fall 2023. The workshop congregated experts from diverse fields including social science, medicine, public health, statistics, computer science, and education, to tackle the unique obstacles each discipline faces in extrapolating experimental findings. Our study presents three key contributions: we integrate ongoing efforts, highlighting methodological synergies across fields; provide an exhaustive review of generalizability and transportability based on the workshop’s discourse; and identify persistent hurdles while suggesting avenues for future research. By doing so, this paper aims to enhance the collective understanding of the generalizability and transportability of causal effects, fostering cross-disciplinary collaboration and offering valuable insights for researchers working on refining and applying causal inference methods.

Sensitivity analysis for the generalization of experimental results
Journal of the Royal Statistical Society: Series A (2024)

Article ArXiv

Randomized controlled trials (RCT’s) allow researchers to estimate causal effects in an experimental sample with minimal identifying assumptions. However, to generalize or transport a causal effect from an RCT to a target population, researchers must adjust for a set of treatment effect moderators. In practice, it is impossible to know whether the set of moderators has been properly accounted for. In the following paper, I propose a two parameter sensitivity analysis for generalizing or transporting experimental results using weighted estimators. The contributions in the paper are three-fold. First, I show that the sensitivity parameters are scale-invariant and standardized, and introduce an estimation approach for researchers to simultaneously account for the bias in their estimates from omitting a moderator, as well as potential changes to their inference. Second, I propose several tools researchers can use to perform sensitivity analysis: (1) different numerical measures to summarize the uncertainty in an estimated effect to unobserved confounding; (2) graphical summary tools for researchers to visualize the sensitivity in their estimated effects, as the confounding strength of the omitted variable changes; and (3) a formal benchmarking approach for researchers to estimate potential sensitivity parameter values using existing data. Finally, I demonstrate that the proposed framework can be easily extended to the class of doubly robust, augmented weighted estimators. The sensitivity analysis framework is applied to a set of Jobs Training Program experiments.

Leveraging population outcomes to improve the generalization of experimental results
with Naoki Egami, Erin Hartman, and Luke Miratrix
Annals of Applied Statistics (2023)

Article ArXiv

Randomized control trials are often considered the gold standard in causal inference due to their high internal validity. Despite its importance, generalizing experimental results to a target population is challenging in social and biomedical sciences. Recent papers clarify the assumptions necessary for generalization and develop various weighting estimators for the population average treatment effect (PATE). However, in practice, many of these methods result in large variance and little statistical power, thereby limiting the value of the PATE inference. In this article, we propose post-residualized weighting, in which information about the outcome measured in the observational population data is used to improve the efficiency of many existing popular methods without making additional assumptions. We empirically demonstrate the efficiency gains through simulations and apply our proposed method to a set of jobs training program experiments.

Improving precision in the design and analysis of experiments with non-compliance
with Erin Hartman
Political Science Research and Methods (2023)

Article Code

Even in the best-designed experiment, noncompliance with treatment assignment can complicate analysis. Under one-way noncompliance, researchers typically rely on an instrumental variables approach, under an exclusion restriction assumption, to identify the complier average causal effect (CACE). This approach suffers from high variance, particularly when the experiment has a low compliance rate. The following paper suggests blocking designs that can help overcome precision losses in the face of high rates of noncompliance in experiments when a placebo-controlled design is infeasible. We also introduce the principal ignorability assumption and a class of principal score weighted estimators, which are largely absent from the experimental political science literature. We then introduce the ''block principal ignorability'' assumption which, when combined with a blocking design, suggests a simple difference-in-means estimator for estimating the CACE. We show that blocking can improve precision of both IV and principal score weighting approaches, and further show that our simple, design-based solution has superior performance to both principal score weighting and instrumental variables under blocking. Finally, in a re-evaluation of the Gerber, Green, and Nickerson (2003) study, we find that blocked, principal ignorability approaches to estimation of the CACE, including our blocked difference-in-means and principal score weighting estimators, result in confidence intervals roughly half the size of traditional instrumental variable approaches.

Sensitivity Analysis for Survey Weighting
with Erin Hartman
Political Analysis (2023)

Article ArXiv Code

Survey weighting allows researchers to account for bias in survey samples, due to unit nonresponse or convenience sampling, using measured demographic covariates. Unfortunately, in practice, it is impossible to know whether the estimated survey weights are sufficient to alleviate concerns about bias due to unobserved confounders or incorrect functional forms used in weighting. In the following paper, we propose two sensitivity analyses for the exclusion of important covariates: (1) a sensitivity analysis for partially observed confounders (i.e., variables measured across the survey sample, but not the target population) and (2) a sensitivity analysis for fully unobserved confounders (i.e., variables not measured in either the survey or the target population). We provide graphical and numerical summaries of the potential bias that arises from such confounders, and introduce a benchmarking approach that allows researchers to quantitatively reason about the sensitivity of their results. We demonstrate our proposed sensitivity analyses using state-level 2020 U.S. Presidential Election polls.

Higher Moments for Optimal Balance Weighting in Causal Estimation
with Brian Vegetabile, Lane Burgette, Claude Setodji, and Beth Ann Griffin
Epidemiology (2022)

Article

We expand upon a simulation study that compared three promising methods for estimating weights for assessing the average treatment effect on the treated for binary treatments: generalized boosted models, covariate-balancing propensity scores, and entropy balance. The original study showed that generalized boosted models can outperform covariate-balancing propensity scores, and entropy balance when there are likely to be non-linear associations in both the treatment assignment and outcome models and when the other two models are fine-tuned to obtain balance only on first-order moments. We explore the potential benefit of using higher-order moments in the balancing conditions for covariate-balancing propensity scores and entropy balance. Our findings showcase that these two models should, by default, include higher order moments and focusing only on first moments can result in substantial bias in estimated treatment effect estimates from both models that could be avoided using higher moments.

[Dec. 2024]		My paper with Yi Zhang and Kosuke Imai on minimax regret estimation for generalizing heterogeneous treatment effects is now available on ArXiv (link).
[Sept. 2024]		My paper on a overlap violations in external validity is forthcoming in the Annals of Applied Statistics.
[Aug. 2024]		My paper with Harsh Parikh on recent advances in the generalizability literature will be appearing in Harvard Data Science Review.
[Jul. 2024]		My paper with Sam Pimentel on variance-based sensitivity models for weighted estimators will be appearing in Biometrika!
[Apr. 2024]		My paper on a sensitivity framework for considering overlap violations in external validity is now available on ArXiv (link).

+ Welcome!

+ Research

+ Teaching