Statistical Inference for Additive Monotone Models under the Fixed Lattice Design

Abstract: We study statistical inference for least squares estimators (LSEs) in additive monotone models under a general fixed lattice design. We establish joint limiting distributions for the LSEs and show that the estimators of different additive components are asymptotically independent. The form of the limiting distribution of each component is determined by how fast the number of design points along the corresponding coordinate grows relative to the total sample size n. Apart from this growth rate, the limit depends only on the noise level and, in the non-Gaussian regimes, on the local derivative of the component. In particular, the limit does not depend on the dimension of the model. When the number of design points along a coordinate grows faster than n^1/3, we construct tuning-free pointwise confidence intervals based on a pivotal limiting distribution, and we validate the theory in numerical simulations. We further show that the block-size normalization underlying these intervals fails to be pivotal at the critical growth rate n^1/3. We also prove a switching lemma, of independent interest, that simplifies the derivation of limiting distributions in isotonic regression.