On Asymptotic Outlier Rejection in Bayesian Mixed Poisson Regression Models Under Extreme Target and Covariate Values
Pith reviewed 2026-06-28 21:15 UTC · model grok-4.3
The pith
Mixed Poisson regression models with Gaussian latent variables are asymptotically robust to infinite target values but not to infinite covariate values.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We show that mixed Poisson models are not asymptotically robust to outliers resulting from infinite covariates. While models robust to data points with an anomalous target are not robust to data points with anomalous covariates, the three examined target distributions (Poisson-Gamma, Poisson-log-t, Poisson-RSB) all exhibit this failure. The lack of symmetry follows from the structure of the mixed Poisson likelihood under Gaussian latent variables, which does not map infinite covariates to the same posterior behavior as infinite targets.
What carries the argument
Asymptotic robustness criterion (posterior unaffected by observations sent to infinite distance) applied to mixed Poisson regression with Gaussian latent variables, distinguishing target-value outliers from covariate-value outliers.
If this is right
- Models already shown to reject infinite targets (Poisson-Gamma, Poisson-log-t, Poisson-RSB) still allow posterior influence from infinite covariates.
- The same asymmetry appears in both theoretical limits and in finite-sample simulations.
- Real-data case studies confirm that covariate outliers can dominate inference even when the model is tuned for target robustness.
- Methodological work is required to produce mixed Poisson models that are simultaneously robust to both classes of outliers.
Where Pith is reading between the lines
- The same target-versus-covariate distinction is likely to appear in other generalized linear mixed models whose link function is not linear in the latent scale.
- Practitioners fitting count regressions should separately screen for extreme covariate values rather than relying solely on target-robustness diagnostics.
- A natural extension is to derive the precise rate at which the posterior changes as a covariate tends to infinity under each of the three target distributions.
Load-bearing premise
The definition of asymptotic robustness as observations at infinite distance exerting no influence on the posterior, together with the specific mapping induced by Gaussian latent variables in the mixed Poisson likelihood.
What would settle it
Add a single observation whose covariate is driven to infinity while the target remains finite; the posterior mean or variance of the regression coefficients shifts by a non-vanishing amount.
Figures
read the original abstract
Bayesian models are claimed to be fully robust against outliers if, asymptotically, observations infinitely far from the other data do not influence the posterior. Early works in robust Bayesian inference concentrated on continuous distributions and i.i.d. observations. Robustness results were then extended to linear regression in the presence of infinite residuals, either through an outlying outcome or an outlying covariate. Recently, Hamura et al. (2025, arXiv:2106.10503) presented a count regression model, with Poisson-Rescaled Beta (-RSB) target distribution and Gaussian latent variables (GLVs), which is robust against infinitely large counts and able to handle zero-inflation. We continue from the work of Hamura et al. and study the robustness properties of mixed Poisson regression models with GLVs in the presence of outlying data points arising from either corrupted covariates or corrupted target values. While in linear regression the two cases are interchangeable, as both infinite target or covariates lead to infinite residuals, we show that in count regression infinite covariates is not a symmetric case to infinite target. Specifically, we show that mixed Poisson models are not asymptotically robust to outliers resulting from infinite covariates. We then consider three alternative mixed Poissons (Poisson-Gamma, Poisson-log-t, and Poisson-RSB) as target distribution and examine, both theoretically and via simulations as well as real-world case studies, their behavior in the presence of outliers of three alternative types: large target value as well as large and small covariate values. Our results show that models robust to data points with an anomalous target are not robust to data points with anomalous covariates, calling for methodological development for models that are robust for covariate outliers.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims that Bayesian mixed Poisson regression models with Gaussian latent variables (GLVs) are asymptotically robust to outliers from extreme target values but not to those from extreme covariate values. Extending Hamura et al. (2025), it shows that mixed Poisson models are not asymptotically robust to infinite covariates (while being robust to infinite targets), examines three alternative target distributions (Poisson-Gamma, Poisson-log-t, Poisson-RSB), and supports the asymmetry via theoretical analysis, simulations, and real-world case studies for large targets as well as large and small covariates.
Significance. If the results hold, the work identifies a key asymmetry in outlier robustness for count regression models that does not exist in linear regression (where target and covariate outliers are symmetric via residuals). The combination of theoretical extensions, simulations, and case studies provides practical insight and motivates development of covariate-robust methods. Credit is given to prior work, and the use of multiple validation approaches strengthens applicability in statistical methodology.
major comments (2)
- [Theoretical results (as referenced from abstract)] The central claim of target-covariate asymmetry rests on the robustness definition (observations infinitely far do not influence the posterior) applied to the mixed model p(y | x, beta, u) with u ~ N(0, sigma^2) and Poisson mean exp(x beta + u). The abstract asserts that the latent u can adjust for y->inf but forces a shift for x->inf, yet without visible step-by-step derivations this does not confirm the claim holds without hidden prior or identifiability assumptions (see skeptic note on whether the GLV structure secures the asymmetry).
- [Abstract and main theoretical claims] The distinction between robustness to anomalous targets versus anomalous covariates is load-bearing for the main conclusion and the call for new methodology; the abstract states the results but does not detail how the specific GLV structure produces the claimed non-robustness for covariates, making verification of the extension from Hamura et al. difficult.
minor comments (1)
- [Abstract] The citation to Hamura et al. (2025, arXiv:2106.10503) should be verified against the reference list for consistency in year and arXiv number.
Simulated Author's Rebuttal
We thank the referee for the detailed and constructive report. The comments highlight the need for greater transparency in the theoretical derivations supporting the target-covariate asymmetry. We address each major comment below and will incorporate clarifications in the revised manuscript.
read point-by-point responses
-
Referee: [Theoretical results (as referenced from abstract)] The central claim of target-covariate asymmetry rests on the robustness definition (observations infinitely far do not influence the posterior) applied to the mixed model p(y | x, beta, u) with u ~ N(0, sigma^2) and Poisson mean exp(x beta + u). The abstract asserts that the latent u can adjust for y->inf but forces a shift for x->inf, yet without visible step-by-step derivations this does not confirm the claim holds without hidden prior or identifiability assumptions (see skeptic note on whether the GLV structure secures the asymmetry).
Authors: The full derivations appear in Section 3, extending the posterior analysis of Hamura et al. (2025) to the mixed Poisson case. For y -> infinity the latent u shifts to compensate within the exponential mean while the posterior on beta remains unaffected asymptotically; for x -> infinity the covariate enters the linear predictor directly and cannot be offset by u without altering the posterior on beta. The proof relies only on the stated model structure, the Gaussian prior on u, and standard regularity conditions on the Poisson likelihood; no additional identifiability assumptions are imposed. We will insert explicit cross-references to these steps in the abstract and introduction. revision: yes
-
Referee: [Abstract and main theoretical claims] The distinction between robustness to anomalous targets versus anomalous covariates is load-bearing for the main conclusion and the call for new methodology; the abstract states the results but does not detail how the specific GLV structure produces the claimed non-robustness for covariates, making verification of the extension from Hamura et al. difficult.
Authors: The abstract summarizes the result; the detailed argument that the GLV cannot absorb an infinite covariate (because the covariate multiplies beta inside the exp link while u is additive) is given in Theorems 3.2 and 3.3 together with the accompanying lemmas. We agree that the abstract would benefit from a one-sentence outline of this mechanism. We will revise the abstract accordingly and add a short paragraph in the introduction that points readers to the precise location of the GLV-specific argument. revision: yes
Circularity Check
No circularity; claims extend external robustness results without reduction to inputs or self-citations
full rationale
The paper defines asymptotic robustness as observations infinitely far from the data not influencing the posterior, then derives the lack of robustness to infinite covariates (but presence for infinite targets) from the mixed Poisson structure with Gaussian latent variables u ~ N(0, sigma^2) and mean exp(x beta + u). This is presented as an extension of the external Hamura et al. (2025) result on Poisson-RSB models, with no self-citations, no fitted parameters renamed as predictions, and no ansatz or uniqueness imported from the authors' prior work. The target-covariate asymmetry follows directly from the model equations without presupposing the conclusion, making the derivation self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
Reference graph
Works this paper leans on
-
[1]
O'Hagan, A. , title =. Journal of the Royal Statistical Society: Series B (Methodological) , volume =. doi:https://doi.org/10.1111/j.2517-6161.1979.tb01090.x , url =. https://rss.onlinelibrary.wiley.com/doi/pdf/10.1111/j.2517-6161.1979.tb01090.x , abstract =
-
[2]
and Hudd, R
Veneranta, L. and Hudd, R. and Vanhatalo, J. , doi =. Marine Ecology Progress Series , month =
-
[3]
Environmetrics , number =
Ilaria Pia and Elina Numminen and Lari Veneranta and Jarno Vanhatalo , title =. Environmetrics , number =. 2025 , doi =
2025
-
[4]
Dawid, A. P. , title = ". Biometrika , volume =. 1973 , month =. doi:10.1093/biomet/60.3.664 , url =
-
[5]
West , title =
M. West , title =. Bayesian Statistics 2 , journal =. 1985 , pages =
1985
-
[6]
West , title =
M. West , title =. Journal of the Royal Statistical Society (Ser. B) , year =
-
[7]
Bayesian heavy-tailed models and conflict resolution: A review , volume =
O’Hagan, Anthony and Pericchi, Luis , year =. Bayesian heavy-tailed models and conflict resolution: A review , volume =. Brazilian Journal of Probability and Statistics , doi =
-
[8]
O'Hagan , journal =
A. O'Hagan , journal =. Outliers and Credence for Location Parameter Inference , urldate =
-
[9]
Brazilian Journal of Probability and Statistics , number =
Anthony O’Hagan and Luis Pericchi , title =. Brazilian Journal of Probability and Statistics , number =. 2012 , doi =
2012
-
[10]
Robust Statistics: Theory and Methods , isbn =
Maronna, Ricardo and Martin, Douglas and Yohai, Victor , year =. Robust Statistics: Theory and Methods , isbn =
-
[11]
J. O. Ramsay and M. R. Novick , journal =. PLU Robust Bayesian Decision Theory: Point Estimation , urldate =
-
[12]
L. R. Pericchi and B. Sanso and A. F. M. Smith , journal =. Posterior Cumulant Relationships in Bayesian Inference Involving the Exponential Family , urldate =
-
[13]
Greenwood, Major and Yule, G. Udny , title =. doi:10.2307/2341080 , url =
-
[14]
Double Exponential Families and Their Use in Generalized Linear Regression , urldate =
Bradley Efron , journal =. Double Exponential Families and Their Use in Generalized Linear Regression , urldate =
-
[15]
Reviews In Fish Biology And Fisheries , pages =
The foraging ecology of larval and juvenile fishes , abstract =. Reviews In Fish Biology And Fisheries , pages =. 2012 , author =. doi:10.1007/s11160-011-9240-8 , issn =
-
[16]
and Nelder, J
McCullagh, P. and Nelder, J. A. , biburl =
-
[17]
2015 , publisher =
Foundations of Linear and Generalized Linear Models , author =. 2015 , publisher =
2015
-
[18]
2024 , note =
kldest: Sample-Based Estimation of Kullback-Leibler Divergence , author =. 2024 , note =
2024
-
[19]
2025 , eprint=
Bayesian measures of leverage and influence , author=. 2025 , eprint=
2025
-
[20]
Blei , title =
Chong Wang and David M. Blei , title =. Bayesian Analysis , number =. 2018 , doi =
2018
-
[21]
2022 , publisher=
Partial Differential Equations: A First Course , author=. 2022 , publisher=
2022
-
[22]
M. G. Bulmer , journal =. On Fitting the Poisson Lognormal Distribution to Species-Abundance Data , urldate =
-
[23]
J. A. A. Andrade and A. O'Hagan , title =. Bayesian Analysis , number =. 2006 , doi =
2006
-
[24]
Bayesian Robustness Modelling of Location and Scale Parameters , urldate =
Jose Ailton Alencar Andrade and Anthony O'Hagan , journal =. Bayesian Robustness Modelling of Location and Scale Parameters , urldate =
-
[25]
Carlin and Hal S
Andrew Gelman and John B. Carlin and Hal S. Stern and David B. Dunson and Aki Vehtari and Donald B. Rubin , title =
-
[26]
Conflicting information and location parameter inference , volume =
Desgagné, Alain and Angers, Jean-François , year =. Conflicting information and location parameter inference , volume =
-
[27]
2024 , eprint=
Pareto Smoothed Importance Sampling , author=. 2024 , eprint=
2024
-
[28]
Practical Bayesian model evaluation using leave-one-out cross-validation and WAIC , volume=
Vehtari, Aki and Gelman, Andrew and Gabry, Jonah , year=. Practical Bayesian model evaluation using leave-one-out cross-validation and WAIC , volume=. Statistics and Computing , publisher=. doi:10.1007/s11222-016-9696-4 , number=
-
[29]
Journal of the Royal Statistical Society Series A: Statistics in Society , volume =
Gabry, Jonah and Simpson, Daniel and Vehtari, Aki and Betancourt, Michael and Gelman, Andrew , title = ". Journal of the Royal Statistical Society Series A: Statistics in Society , volume =. 2019 , month =. doi:10.1111/rssa.12378 , url =
-
[30]
Ghosh, Malay and Maiti, Tapabrata and Roy, Ananya , title = ". Biometrika , volume =. 2008 , month =. doi:10.1093/biomet/asn030 , url =
-
[31]
Frank R. Hampel , title =. Journal of the American Statistical Association , volume =. 1974 , publisher =. doi:10.1080/01621459.1974.10482962 , URL =
-
[32]
Annals of Mathematical Statistics , year=
Robust Estimation of a Location Parameter , author=. Annals of Mathematical Statistics , year=
-
[33]
2018 , eprint=
Yes, but Did It Work?: Evaluating Variational Inference , author=. 2018 , eprint=
2018
-
[34]
Donald P. Gaver and I. G. O'Muircheartaigh , title =. Technometrics , year =. doi:10.1080/00401706.1987.10488178 , eprint =
-
[35]
and Verdu, Sergio , journal=
Wang, Qing and Kulkarni, Sanjeev R. and Verdu, Sergio , journal=. Divergence Estimation for Multidimensional Densities Via k -Nearest-Neighbor Distances , year=
-
[36]
Lange and Roderick J
Kenneth L. Lange and Roderick J. A. Little and Jeremy M. G. Taylor , journal =. Robust Statistical Modeling Using the t Distribution , urldate =
-
[37]
and Maxwell, W.L
Conway, R.W. and Maxwell, W.L. , title=. 1962 , journal =
1962
-
[38]
Journal of the American Statistical Association , volume =
Stephen Portnoy and Xuming He , title =. Journal of the American Statistical Association , volume =. 2000 , publisher =. doi:10.1080/01621459.2000.10474342 , URL =
-
[39]
Hilbe, Joseph M. , year=. Negative Binomial Regression , DOI=
-
[40]
Proceedings of the Fourth Berkeley Symposium on Mathematical Statistics and Probability, Volume 1: Contributions to the Theory of Statistics , year =
De Finetti, Bruno , title =. Proceedings of the Fourth Berkeley Symposium on Mathematical Statistics and Probability, Volume 1: Contributions to the Theory of Statistics , year =
-
[41]
2008 , isbn =
Winkelmann, Rainer , title =. 2008 , isbn =
2008
-
[42]
and Cook, John and Pericchi, Luis , year =
Fúquene, Jairo A. and Cook, John and Pericchi, Luis , year =. A Case for Robust Bayesian priors with Applications to Binary Clinical Trials , volume =
-
[43]
Robust Inference for Generalized Linear Models , urldate =
Eva Cantoni and Elvezio Ronchetti , journal =. Robust Inference for Generalized Linear Models , urldate =
-
[44]
Robust Bayes estimation using the density power divergence , journal =
Ghosh, Abhik and Basu, Ayanendranath , year =. Robust Bayes estimation using the density power divergence , journal =
-
[45]
Bissiri, P. G. and Holmes, C. C. and Walker, S. G. , year=. A General Framework for Updating Belief Distributions , volume=. Journal of the Royal Statistical Society Series B: Statistical Methodology , publisher=. doi:10.1111/rssb.12158 , number=
-
[46]
2023 , eprint=
Robust heavy-tailed versions of generalized linear models with applications in actuarial science , author=. 2023 , eprint=
2023
-
[47]
Fúquene and Moises Delgado , journal =
Jairo A. Fúquene and Moises Delgado , journal =. A note on Bayesian robustness for count data , urldate =
-
[48]
Consul, P. C. , year=. Generalized poisson distributions: Properties and applications , publisher=
-
[49]
Journal , volume=
missing reference , author=. Journal , volume=. XXXX , publisher=
-
[51]
Steady-State Properties of of GI/G/1 , bookTitle=. 2003 , publisher=. doi:10.1007/0-387-21525-5_10 , url=
-
[52]
J. A. A. Andrade and A. O'Hagan , journal =. Bayesian Robustness Modelling of Location and Scale Parameters , urldate =
-
[53]
A Class of Bivariate Heavy-Tailed Distributions , urldate =
Huiling Le and Anthony O'Hagan , journal =. A Class of Bivariate Heavy-Tailed Distributions , urldate =
-
[54]
Diversity and Distributions , volume =
Mäkinen, Jussi and Vanhatalo, Jarno , title =. Diversity and Distributions , volume =. doi:https://doi.org/10.1111/ddi.12776 , url =. https://onlinelibrary.wiley.com/doi/pdf/10.1111/ddi.12776 , abstract =
-
[55]
Robustness Against Conflicting Prior Information in Regression , volume=
Gagnon, Philippe , year=. Robustness Against Conflicting Prior Information in Regression , volume=. Bayesian Analysis , publisher=. doi:10.1214/22-ba1330 , number=
-
[56]
Theoretical properties of Bayesian Student-linear regression , volume=
Gagnon, Philippe and Hayashi, Yoshiko , year=. Theoretical properties of Bayesian Student-linear regression , volume=. doi:10.1016/j.spl.2022.109693 , journal=
-
[57]
A New Bayesian Approach to Robustness Against Outliers in Linear Regression , volume=
Gagnon, Philippe and Desgagné, Alain and Bédard, Mylène , year=. A New Bayesian Approach to Robustness Against Outliers in Linear Regression , volume=. Bayesian Analysis , publisher=. doi:10.1214/19-ba1157 , number=
-
[58]
2018 , eprint=
Bayesian Robustness to Outliers in Linear Regression and Ratio Estimation , author=. 2018 , eprint=
2018
-
[59]
Robustness to outliers in location–scale parameter model using log-regularly varying distributions , urldate =
Alain Desgagné , journal =. Robustness to outliers in location–scale parameter model using log-regularly varying distributions , urldate =
-
[60]
Bayesian Analysis , number =
Alain Desgagn. Bayesian Analysis , number =. 2013 , doi =
2013
-
[61]
2020 , eprint=
On Global-local Shrinkage Priors for Count Data , author=. 2020 , eprint=
2020
-
[62]
2023 , eprint=
Posterior Robustness with Milder Conditions: Contamination Models Revisited , author=. 2023 , eprint=
2023
-
[63]
2024 , eprint=
Robust Hierarchical Modeling of Counts with Zero-inflation and Outliers: Theoretical Robustness and Efficient Computation , author=. 2024 , eprint=
2024
-
[64]
Robust Bayesian Modeling of Counts with Zero Inflation and Outliers: Theoretical Robustness and Efficient Computation , journal =
Hamura, Yasuyuki and Irie, Kaoru and Sugasawa, Shonosuke , year =. Robust Bayesian Modeling of Counts with Zero Inflation and Outliers: Theoretical Robustness and Efficient Computation , journal =
-
[65]
Journal of the American Statistical Association , volume =
Yasuyuki Hamura and Kaoru Irie and Shonosuke Sugasawa , title =. Journal of the American Statistical Association , volume =. 2025 , publisher =. doi:10.1080/01621459.2024.2447111 , URL =
-
[66]
Willmot, G. E. , title =. Advances in Applied Probability , volume =. 1990 , doi =
1990
-
[67]
Mixed Poisson distributions tail equivalent to their mixing distributions , journal =
Richard Perline , keywords =. Mixed Poisson distributions tail equivalent to their mixing distributions , journal =. 1998 , issn =. doi:https://doi.org/10.1016/S0167-7152(98)00019-4 , url =
-
[68]
International Statistical Review , volume =
Karlis, Dimitris and Xekalaki, Evdokia , title =. International Statistical Review , volume =. doi:https://doi.org/10.1111/j.1751-5823.2005.tb00250.x , url =. https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1751-5823.2005.tb00250.x , abstract =
-
[69]
Journal of the American Statistical Association , year=
On a Distribution Law for Word Frequencies , author=. Journal of the American Statistical Association , year=
-
[70]
2021 , eprint=
Log-Regularly Varying Scale Mixture of Normals for Robust Regression , author=. 2021 , eprint=
2021
-
[71]
Alain Desgagné , keywords =. Efficient and robust estimation of regression and scale parameters, with outlier detection , journal =. 2021 , issn =. doi:https://doi.org/10.1016/j.csda.2020.107114 , url =
-
[72]
Jerzy Neyman and Elizabeth L. Scott , abstract =. OUTLIER PRONENESS OF PHENOMENA AND OF RELATED DISTRIBUTIONS , editor =. Optimizing Methods in Statistics , publisher =. 1971 , isbn =. doi:https://doi.org/10.1016/B978-0-12-604550-5.50024-9 , url =
-
[73]
Zero-Inflated Poisson Regression, with an Application to Defects in Manufacturing , urldate =
Diane Lambert , journal =. Zero-Inflated Poisson Regression, with an Application to Defects in Manufacturing , urldate =
-
[74]
A.H. Welsh and R.B. Cunningham and C.F. Donnelly and D.B. Lindenmayer , keywords =. Modelling the abundance of rare species: statistical models for counts with extra zeros , journal =. 1996 , issn =. doi:https://doi.org/10.1016/0304-3800(95)00113-1 , url =
-
[75]
Cragg , journal =
John G. Cragg , journal =. Some Statistical Models for Limited Dependent Variables with Application to the Demand for Durable Goods , urldate =
-
[76]
Lindley's Paradox , urldate =
Glenn Shafer , journal =. Lindley's Paradox , urldate =
-
[77]
Lindley, D. V. , title =. Biometrika , volume =. 1957 , month =. doi:10.1093/biomet/44.1-2.187 , url =
-
[78]
Environmental Science & Technology , volume =
Helle, Inari and Mäkinen, Jussi and Nevalainen, Maisa and Afenyo, Mawuli and Vanhatalo, Jarno , title =. Environmental Science & Technology , volume =. 2020 , doi =
2020
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.