Expander Framework for Generating High-Dimensional GLM Gradient and Hessian from Low-Dimensional Base Distributions: R Package RegressionFactory

The R package RegressionFactory provides expander functions for constructing the high-dimensional gradient vector and Hessian matrix of the log-likelihood function for generalized linear models (GLMs), from the lower-dimensional base-distribution derivatives. The software follows a modular implementation using the chain rule of derivatives. Such modularity offers a clear separation of case-specific components (base distribution functional form and link functions) from common steps (e.g., matrix algebra operations needed for expansion) in calculating log-likelihood derivatives. In doing so, RegressionFactory offers several advantages: 1) It provides a fast and convenient method for constructing log-likelihood and its derivatives by requiring only the low-dimensional, base-distribution derivatives, 2) The accompanying definiteness-invariance theorem allows researchers to reason about the negative-definiteness of the log-likelihood Hessian in the much lower-dimensional space of the base distributions, 3) The factorized, abstract view of regression suggests opportunities to generate novel regression models, and 4) Computational techniques for performance optimization can be developed generically in the abstract framework and be readily applicable across all the specific regression instances. We expect RegressionFactory to facilitate research and development on optimization and sampling techniques for GLM log-likelihoods as well as construction of composite models from GLM lego blocks, such as Hierarchical Bayesian models.