No Arabic abstract
The problem of obtaining optimal projections for performing discriminant analysis with Gaussian class densities is studied. Unlike in most existing approaches to the problem, the focus of the optimisation is on the multinomial likelihood based on posterior probability estimates, which directly captures discriminability of classes. In addition to the more commonly considered problem, in this context, of classification, the unsupervised clustering counterpart is also considered. Finding optimal projections offers utility for dimension reduction and regularisation, as well as instructive visualisation for better model interpretability. Practical applications of the proposed approach show considerable promise for both classification and clustering. Code to implement the proposed method is available in the form of an R package from https://github.com/DavidHofmeyr/OPGD.
Bayesian approaches are appealing for constrained inference problems in allowing a probabilistic characterization of uncertainty, while providing a computational machinery for incorporating complex constraints in hierarchical models. However, the usual Bayesian strategy of placing a prior on the constrained space and conducting posterior computation with Markov chain Monte Carlo algorithms is often intractable. An alternative is to conduct inference for a less constrained posterior and project samples to the constrained space through a minimal distance mapping. We formalize and provide a unifying framework for such posterior projections. For theoretical tractability, we initially focus on constrained parameter spaces corresponding to closed and convex subsets of the original space. We then consider non-convex Stiefel manifolds. We provide a general formulation of the projected posterior and show that it can be viewed as an update of a data-dependent prior with the likelihood for particular classes of priors and likelihood functions. We also show that asymptotic properties of the unconstrained posterior are transferred to the projected posterior. Posterior projections are illustrated through multiple examples, both in simulation studies and real data applications.
Causal inference of treatment effects is a challenging undertaking in it of itself; inference for sequential treatments leads to even more hurdles. In precision medicine, one additional ambitious goal may be to infer about effects of dynamic treatment regimes (DTRs) and to identify optimal DTRs. Conventional methods for inferring about DTRs involve powerful semi-parametric estimators. However, these are not without their strong assumptions. Dynamic Marginal Structural Models (MSMs) are one semi-parametric approach used to infer about optimal DTRs in a family of regimes. To achieve this, investigators are forced to model the expected outcome under adherence to a DTR in the family; relatively straightforward models may lead to bias in the optimum. One way to obviate this difficulty is to perform a grid search for the optimal DTR. Unfortunately, this approach becomes prohibitive as the complexity of regimes considered increases. In recently developed Bayesian methods for dynamic MSMs, computational challenges may be compounded by the fact that at each grid point, a posterior mean must be calculated. We propose a manner by which to alleviate modelling difficulties for DTRs by using Gaussian process optimization. More precisely, we show how to pair this optimization approach with robust estimators for the causal effect of adherence to a DTR to identify optimal DTRs. We examine how to find the optimum in complex, multi-modal settings which are not generally addressed in the DTR literature. We further evaluate the sensitivity of the approach to a variety of modeling assumptions in the Gaussian process.
We propose optimal observables for Gaussian illumination to maximize the signal-to-noise ratio, which minimizes the discrimination error between the presence and absence of a low-reflectivity target using Gaussian states. The optimal observables dominantly consist of off-diagonal components of output states, which is implemented with feasible setups. In the quantum regime using a two-mode squeezed vacuum state, the receiver implemented with heterodyne detections outperforms the other feasible receivers, which asymptotically improves the error probability exponent by a factor of two over the classical state bound. In the classical regime using coherent or thermal states, the receiver implemented with photon number difference measurement asymptotically approaches its bound.
Generalized Gaussian processes (GGPs) are highly flexible models that combine latent GPs with potentially non-Gaussian likelihoods from the exponential family. GGPs can be used in a variety of settings, including GP classification, nonparametric count regression, modeling non-Gaussian spatial data, and analyzing point patterns. However, inference for GGPs can be analytically intractable, and large datasets pose computational challenges due to the inversion of the GP covariance matrix. We propose a Vecchia-Laplace approximation for GGPs, which combines a Laplace approximation to the non-Gaussian likelihood with a computationally efficient Vecchia approximation to the GP, resulting in a simple, general, scalable, and accurate methodology. We provide numerical studies and comparisons on simulated and real spatial data. Our methods are implemented in a freely available R package.
Neural topic models have triggered a surge of interest in extracting topics from text automatically since they avoid the sophisticated derivations in conventional topic models. However, scarce neural topic models incorporate the word relatedness information captured in word embedding into the modeling process. To address this issue, we propose a novel topic modeling approach, called Variational Gaussian Topic Model (VaGTM). Based on the variational auto-encoder, the proposed VaGTM models each topic with a multivariate Gaussian in decoder to incorporate word relatedness. Furthermore, to address the limitation that pre-trained word embeddings of topic-associated words do not follow a multivariate Gaussian, Variational Gaussian Topic Model with Invertible neural Projections (VaGTM-IP) is extended from VaGTM. Three benchmark text corpora are used in experiments to verify the effectiveness of VaGTM and VaGTM-IP. The experimental results show that VaGTM and VaGTM-IP outperform several competitive baselines and obtain more coherent topics.