1,178 research outputs found
Bounding Optimality Gap in Stochastic Optimization via Bagging: Statistical Efficiency and Stability
We study a statistical method to estimate the optimal value, and the
optimality gap of a given solution for stochastic optimization as an assessment
of the solution quality. Our approach is based on bootstrap aggregating, or
bagging, resampled sample average approximation (SAA). We show how this
approach leads to valid statistical confidence bounds for non-smooth
optimization. We also demonstrate its statistical efficiency and stability that
are especially desirable in limited-data situations, and compare these
properties with some existing methods. We present our theory that views SAA as
a kernel in an infinite-order symmetric statistic, which can be approximated
via bagging. We substantiate our theoretical findings with numerical results
Learning and Management for Internet-of-Things: Accounting for Adaptivity and Scalability
Internet-of-Things (IoT) envisions an intelligent infrastructure of networked
smart devices offering task-specific monitoring and control services. The
unique features of IoT include extreme heterogeneity, massive number of
devices, and unpredictable dynamics partially due to human interaction. These
call for foundational innovations in network design and management. Ideally, it
should allow efficient adaptation to changing environments, and low-cost
implementation scalable to massive number of devices, subject to stringent
latency constraints. To this end, the overarching goal of this paper is to
outline a unified framework for online learning and management policies in IoT
through joint advances in communication, networking, learning, and
optimization. From the network architecture vantage point, the unified
framework leverages a promising fog architecture that enables smart devices to
have proximity access to cloud functionalities at the network edge, along the
cloud-to-things continuum. From the algorithmic perspective, key innovations
target online approaches adaptive to different degrees of nonstationarity in
IoT dynamics, and their scalable model-free implementation under limited
feedback that motivates blind or bandit approaches. The proposed framework
aspires to offer a stepping stone that leads to systematic designs and analysis
of task-specific learning and management schemes for IoT, along with a host of
new research directions to build on.Comment: Submitted on June 15 to Proceeding of IEEE Special Issue on Adaptive
and Scalable Communication Network
Approaches for Outlier Detection in Sparse High-Dimensional Regression Models
Modern regression studies often encompass a very large number of potential predictors,
possibly larger than the sample size, and sometimes growing with the sample
size itself. This increases the chances that a substantial portion of the predictors
is redundant, as well as the risk of data contamination. Tackling these problems is
of utmost importance to facilitate scientific discoveries, since model estimates are
highly sensitive both to the choice of predictors and to the presence of outliers. In
this thesis, we contribute to this area considering the problem of robust model selection
in a variety of settings, where outliers may arise both in the response and
the predictors. Our proposals simplify model interpretation, guarantee predictive
performance, and allow us to study and control the influence of outlying cases on
the fit.
First, we consider the co-occurrence of multiple mean-shift and variance-inflation
outliers in low-dimensional linear models. We rely on robust estimation techniques
to identify outliers of each type, exclude mean-shift outliers, and use restricted
maximum likelihood estimation to down-weight and accommodate variance-inflation
outliers into the model fit. Second, we extend our setting to high-dimensional linear
models. We show that mean-shift and variance-inflation outliers can be modeled as
additional fixed and random components, respectively, and evaluated independently.
Specifically, we perform feature selection and mean-shift outlier detection through
a robust class of nonconcave penalization methods, and variance-inflation outlier
detection through the penalization of the restricted posterior mode. The resulting
approach satisfies a robust oracle property for feature selection in the presence of
data contamination – which allows the number of features to exponentially increase
with the sample size – and detects truly outlying cases of each type with asymptotic
probability one. This provides an optimal trade-off between a high breakdown point
and efficiency. Third, focusing on high-dimensional linear models affected by meanshift
outliers, we develop a general framework in which L0-constraints coupled with
mixed-integer programming techniques are used to perform simultaneous feature
selection and outlier detection with provably optimal guarantees. In particular,
we provide necessary and sufficient conditions for a robustly strong oracle property,
where again the number of features can increase exponentially with the sample size,
and prove optimality for parameter estimation and the resulting breakdown point.
Finally, we consider generalized linear models and rely on logistic slippage to perform
outlier detection and removal in binary classification. Here we use L0-constraints
and mixed-integer conic programming techniques to solve the underlying double
combinatorial problem of feature selection and outlier detection, and the framework
allows us again to pursue optimality guarantees.
For all the proposed approaches, we also provide computationally lean heuristic
algorithms, tuning procedures, and diagnostic tools which help to guide the analysis.
We consider several real-world applications, including the study of the relationships
between childhood obesity and the human microbiome, and of the main drivers of
honey bee loss. All methods developed and data used, as well as the source code to
replicate our analyses, are publicly available
Genomic Clinical Trials and Predictive Medicine by Richard M. Simon
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109589/1/insr12085_3.pd
Proximal Point Imitation Learning
This work develops new algorithms with rigorous efficiency guarantees for
infinite horizon imitation learning (IL) with linear function approximation
without restrictive coherence assumptions. We begin with the minimax
formulation of the problem and then outline how to leverage classical tools
from optimization, in particular, the proximal-point method (PPM) and dual
smoothing, for online and offline IL, respectively. Thanks to PPM, we avoid
nested policy evaluation and cost updates for online IL appearing in the prior
literature. In particular, we do away with the conventional alternating updates
by the optimization of a single convex and smooth objective over both cost and
Q-functions. When solved inexactly, we relate the optimization errors to the
suboptimality of the recovered policy. As an added bonus, by re-interpreting
PPM as dual smoothing with the expert policy as a center point, we also obtain
an offline IL algorithm enjoying theoretical guarantees in terms of required
expert trajectories. Finally, we achieve convincing empirical performance for
both linear and neural network function approximation
Problem-driven scenario generation for stochastic programs
Stochastic programming concerns mathematical programming in the presence of uncertainty. In a stochastic program uncertain parameters are modeled as random vectors and one aims to minimize the expectation, or some risk measure, of a loss function. However, stochastic programs are computationally intractable when the underlying uncertain parameters are modeled by continuous random vectors. Scenario generation is the construction of a finite discrete random vector to use within a stochastic program. Scenario generation can consist of the discretization of a parametric probabilistic model, or the direct construction of a discrete distribution. There is typically a trade-off here in the number of scenarios that are used: one must use enough to represent the uncertainty faithfully but not so many that the resultant problem is computationally intractable. Standard scenario generation methods are distribution-based, that is they do not take into account the underlying problem when constructing the discrete distribution. In this thesis we promote the idea of problem-based scenario generation. By taking into account the structure of the underlying problem one may be able to represent uncertainty in a more parsimonious way. The first two papers of this thesis focus on scenario generation for problems which use a tail-risk measure, such as the conditional value-at-risk, focusing in particular on portfolio selection problems. In the final paper we present a constraint driven approach to scenario generation for simple recourse problems, a class of stochastic programs for minimizing the expected shortfall and surplus of some resources with respect to uncertain demands
Statistical Methodologies
Statistical practices have recently been questioned by numerous independent authors, to the extent that a significant fraction of accepted research findings can be questioned. This suggests that statistical methodologies may have gone too far into an engineering practice, with minimal concern for their foundation, interpretation, assumptions, and limitations, which may be jeopardized in the current context. Disguised by overwhelming data sets, advanced processing, and stunning presentations, the basic approach is often intractable to anyone but the analyst. The hierarchical nature of statistical inference, exemplified by Bayesian aggregation of prior and derived knowledge, may also be challenging. Conceptual simplified studies of the kind presented in this book could therefore provide valuable guidance when developing statistical methodologies, but also applying state of the art with greater confidence
Prescriptive Analytics in Electricity Markets
Electricity markets are a clear example of a sector in which decision making plays a crucial role in its daily activity. Moreover, uncertainty is intrinsic to electricity markets and affects most of the tasks that agents operating in them must carry out. Many of these tasks involve decisions characterized by low risk and being addressed periodically. In this thesis, we refer to these tasks as iterative decisions. This thesis applies the aforementioned innovative frameworks for decision making under uncertainty using contextual information in iterative decision making tasks faced daily by electricity market agents.Decision making is critical for any business to survive in a market environment. Examples of decision making tasks are inventory management, resource allocation or portfolio selection. Optimization, understood as the scientific discipline that studies how to solve mathematical programming problems, can help make more efficient decisions in many of these situations. Particularly relevant, because of their frequency and difficulty, are those decisions affected by uncertainty, i.e., in which some of the parameters that precisely determine the optimization problem are unknown when the decision must be made.
Fortunately, the development of information technologies has led to an explosion in the availability of data that can be used to assist decisions affected by uncertainty. However, most of the available historical data do not correspond to the unknown parameter of the problem but originate from other related sources. This subset of data, potentially valuable for obtaining better decisions, is called contextual information. This thesis is framed within a new scientific effort that seeks to exploit the potential of data and, in particular, of contextual information in decision making. To this end, in this thesis, we have developed mathematical frameworks and data-driven optimization models that exploit contextual information to make better decisions in problems characterized by the presence of uncertain parameters
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