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Bishwamittra Ghosh

Ph.D. Candidate

School of Computing, National University of Singapore (NUS)

bghosh@u.nus.edu

[CV] [Research statement]


I am a Ph.D. student in School of Computing, National University of Singapore (NUS). Currently, I am advised by Kuldeep S. Meel. My research is on verifying and improving the safety-critical properties of machine learning, such as fairness and explainability.


I have completed my bachelor's degree (BSc) in Computer Science and Engineering from Bangladesh University of Engineering and Technology (BUET) in September 2017. My undergraduate thesis is on socio-spatial group queries advised by Dr. Mohammed Eunus Ali (accepted in VLDB 2018). Before joining the Ph.D. program, I have been a lecturer at the Department of Computer Science, United International University, Bangladesh.


Publications

2022

[8]

Efficient Learning of Interpretable Classification Rules

Bishwamittra Ghosh, Dmitry Malioutov, Kuldeep S. Meel.

Proceedings of JAIR, 2022.

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Machine learning has become omnipresent with applications in various safety-critical domains such as medical, law, and transportation. In these domains, high-stake decisions provided by machine learning necessitate researchers to design interpretable models, where the prediction is understandable to a human. In interpretable machine learning, rule-based classifiers are particularly effective in representing the decision boundary through a set of rules comprising input features. Examples of such classifiers include decision trees, decision lists, and decision sets. The interpretability of rule-based classifiers is in general related to the size of the rules, where smaller rules are considered more interpretable. To learn such a classifier, the brute-force direct approach is to consider an optimization problem that tries to learn the smallest classification rule that has close to maximum accuracy. This optimization problem is computationally intractable due to its combinatorial nature and thus, the problem is not scalable in large datasets. To this end, in this paper we study the triangular relationship among the accuracy, interpretability, and scalability of learning rule-based classifiers.

The contribution of this paper is an interpretable learning framework IMLI, that is based on maximum satisfiability (MaxSAT) for synthesizing classification rules expressible in proposition logic. IMLI considers a joint objective function to optimize the accuracy and the interpretability of classification rules and learns an optimal rule by solving an appropriately designed MaxSAT query. Despite the progress of MaxSAT solving in the last decade, the straightforward MaxSAT-based solution cannot scale to practical classification datasets containing thousands to millions of samples. Therefore, we incorporate an efficient incremental learning technique inside the MaxSAT formulation by integrating mini-batch learning and iterative rule-learning. The resulting framework learns a classifier by iteratively covering the training data, wherein in each iteration, it solves a sequence of smaller MaxSAT queries corresponding to each mini-batch. In our experiments, IMLI achieves the best balance among prediction accuracy, interpretability, and scalability. For instance, IMLI attains a competitive prediction accuracy and interpretability w.r.t. existing interpretable classifiers and demonstrates impressive scalability on large datasets where both interpretable and non-interpretable classifiers fail. As an application, we deploy IMLI in learning popular interpretable classifiers such as decision lists and decision sets.

[7]

Algorithmic Fairness Verification with Graphical Models

Bishwamittra Ghosh, Debabrota Basu, Kuldeep S. Meel

Proceedings of AAAI, 2022.

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In recent years, machine learning (ML) algorithms have been deployed in safety-critical and high-stake decision-making, where the fairness of algorithms is of paramount importance. Fairness in ML centers on detecting bias towards certain demographic populations induced by an ML classifier and proposes algorithmic solutions to mitigate the bias with respect to different fairness definitions. To this end, several fairness verifiers have been proposed that compute the bias in the prediction of an ML classifier -- essentially beyond a finite dataset -- given the probability distribution of input features. In the context of verifying linear classifiers, existing fairness verifiers are limited by accuracy due to imprecise modelling of correlations among features and scalability due to restrictive formulations of the classifiers as SSAT or SMT formulas or by sampling.

In this paper, we propose an efficient fairness verifier, called FVGM, that encodes the correlations among features as a Bayesian network. In contrast to existing verifiers, FVGM proposes a stochastic subset-sum based approach for verifying linear classifiers. Experimentally, we show that FVGM leads to an accurate and scalable assessment for more diverse families of fairness-enhancing algorithms, fairness attacks, and group/causal fairness metrics than the state-of-the-art. We also demonstrate that FVGM facilitates the computation of fairness influence functions as a stepping stone to detect the source of bias induced by subsets of features.

2021

[6]

Social-Spatial Group Queries with Keywords

Sajid Hasan Apon, Mohammed Eunus Ali, Bishwamittra Ghosh, Timos Sellis

ACM Transactions on Spatial Algorithms and Systems (TSAS), 2021.

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Social networks with location enabling technologies, also known as geo-social networks, allow users to share their location-specific activities and preferences through check-ins. A user in such a geo-social network can be attributed to an associated location (spatial), her preferences as keywords (textual), and the connectivity (social) with her friends. The fusion of social, spatial, and textual data of a large number of users in these networks provide an interesting insight for finding meaningful geo-social groups of users supporting many real-life applications, including activity planning and recommendation systems. In this article, we introduce a novel query, namely, Top-k Flexible Socio-Spatial Keyword-aware Group Query (SSKGQ), which finds the best k groups of varying sizes around different points of interest (POIs), where the groups are ranked based on the social and textual cohesiveness among members and spatial closeness with the corresponding POI and the number of members in the group. We develop an efficient approach to solve the SSKGQ problem based on our theoretical upper bounds on distance, social connectivity, and textual similarity. We prove that the SSKGQ problem is NP-Hard and provide an approximate solution based on our derived relaxed bounds, which run much faster than the exact approach by sacrificing the group quality slightly. Our extensive experiments on real data sets show the effectiveness of our approaches in different real-life settings.

[5]

Justicia: A Stochastic SAT Approach to Formally Verify Fairness

Bishwamittra Ghosh, Debabrota Basu, Kuldeep S. Meel

Proceedings of AAAI, 2021.

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As a technology ML is oblivious to societal good or bad, and thus, the field of fair machine learning has stepped up to propose multiple mathematical definitions, algorithms, and systems to ensure different notions of fairness in ML applications. Given the multitude of propositions, it has become imperative to formally verify the fairness metrics satisfied by different algorithms on different datasets. In this paper, we propose a stochastic satisfiability (SSAT) framework, Justicia, that formally verifies different fairness measures of supervised learning algorithms with respect to the underlying data distribution. We instantiate Justicia on multiple classification and bias mitigation algorithms, and datasets to verify different fairness metrics, such as disparate impact, statistical parity, and equalized odds. Justicia is scalable, accurate, and operates on non-Boolean and compound sensitive attributes unlike existing distribution-based verifiers, such as FairSquare and VeriFair. Being distribution-based by design, Justicia is more robust than the verifiers, such as AIF360, that operate on specific test samples. We also theoretically bound the finite-sample error of the verified fairness measure.

2020

[4]

A MaxSAT-based Framework for Group Testing

Lorenzo Ciampiconi, Bishwamittra Ghosh, Jonathan Scarlett, Kuldeep S. Meel.

Proceedings of AAAI, 2020.

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The success of MaxSAT (maximum satisfiability) solving in recent years has motivated researchers to apply MaxSAT solvers in diverse discrete combinatorial optimization problems. Group testing has been studied as a combinatorial optimization problem, where the goal is to find defective items among a set of items by performing sets of tests on items. In this paper, we propose a MaxSAT-based framework, called MGT, that solves group testing, in particular, the decoding phase of non-adaptive group testing. We extend this approach to the noisy variant of group testing, and propose a compact MaxSAT-based encoding that guarantees an optimal solution. Our extensive experimental results show that MGT can solve group testing instances of 10000 items with 3% defectivity, which no prior work can handle to the best of our knowledge. Furthermore, MGT has better accuracy than the LP-based approach. We also discover an interesting phase transition behavior in the runtime, which reveals the easy-hard-easy nature of group testing.

[3]

Classification Rules in Relaxed Logical Form

Bishwamittra Ghosh, Dmitry Malioutov, Kuldeep S. Meel.

Proceedings of ECAI, 2020.

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Machine learning algorithms that produce rule-based predictions in Conjunctive Normal form (CNF) or in Disjunctive Normal form (DNF) are arguably some of the most interpretable ones. For example, decision set is an interpretable model in practice, that represents the decision function in the form of DNF. In this paper, we consider relaxed definitions of standard OR/AND operators which allow exceptions in the construction of a clause and also in the selection of clauses in a rule. Building on these relaxed definition, we introduce relaxed-CNF rules, which are motivated by the popular usage of checklists in the medical domain and generalizes the widely employed rule representations including CNF, DNF, and decision sets. While the combinatorial structure of relaxed-CNF rules offers exponential succinctness, the naive learning techniques are computationally expensive. To this end, we propose a novel incremental mini-batch learning procedure, called CRR, that employs advances in the Mixed-Integer Linear Programming (MILP) solvers to efficiently learn relaxed-CNF rules. Our experimental analysis demonstrates that CRR can generate relaxed-CNF rules, which are more accurate and sparser compared to the alternative rule-based models.

2019

[2]

IMLI: An Incremental Framework for MaxSAT-Based Learning of Interpretable Classification Rules

Bishwamittra Ghosh, Kuldeep S. Meel.

Proceedings of AAAI/ACM Conference on AI, Ethics, and Society (AIES), 2019.

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The wide adoption of machine learning in the critical domains such as medical diagnosis, law, education had propelled the need for interpretable techniques due to the need for end user to understand the reasoning behind decisions due to learning systems. The computational intractability of interpretable learning led practitioners to design heuristic techniques, which fail to provide sound handles to tradeoff accuracy and interpretability.

Motivated by the success of MaxSAT solvers over the past decade, recently MaxSAT-based approach, called MLIC, was proposed that seeks to reduce the problem of learning interpretable rules expressed in Conjunctive Normal Form (CNF) to a MaxSAT query. While MLIC was shown to achieve accuracy similar to that of other state oft he art black-box classifiers while generating small interpretable CNF formulas, the runtime performance of MLIC is significantly lagging and renders approach unusable in practice. In this context, authors raised the question: Is it possible to achieve the best of both worlds, i.e., a sound framework for interpretable learning that can take advantage of MaxSAT solvers while scaling to real-world instances?

In this paper, we take a step towards answering the above question in affirmation. We propose an incremental approach to MaxSAT based framework that achieves scalable runtime performance via partition-based training methodology. Extensive experiments on benchmarks arising from UCI repository demonstrate that IMLI achieves up to three orders of magnitude runtime improvement without loss of accuracy and interpretability.

2018

[1]

The Flexible Socio Spatial Group Queries

Bishwamittra Ghosh, Mohammed Eunus Ali, Farhana M. Choudhury,

Sajid Hasan Apon, Timos Sellis, Jianxin Li.

Proceedings of the VLDB Endowment (PVLDB), 2018.

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A socio spatial group query finds a group of users who possess strong social connections with each other and have the minimum aggregate spatial distance to a meeting point. Existing studies limit the socio spatial group search to either finding best group of a fixed size for a single meeting location, or a single group of a fixed size w.r.t. multiple meeting locations. However, it is highly desirable to consider multiple meeting locations/POIs in a real-life scenario in order to organize impromptu activities of user groups of various sizes. In this paper, we propose Top k Flexible Socio Spatial Group Query (Top k-FSSGQ) to find and rank the top k groups w.r.t. multiple POIs where each group follows the minimum social connectivity constraints. We devise a ranking function to measure the group score by combining social closeness, spatial distance, and group size, which provides the flexibility of choosing groups of different sizes under different constraints. To effectively process the Top k-FSSGQ, we first develop an Exact approach that ensures early termination of the search based on the derived upper bounds. We prove that the problem is NP-hard, hence we first present a heuristic based approximation algorithm to effectively select members in intermediate solution groups based on the social connectivity of the users. Later we design a Fast Approximate approach based on the relaxed social and spatial bounds, and connectivity constraint heuristic. Experimental studies have verified the effectiveness and efficiency of our proposed approaches on real datasets.

News

Our work on Algorithmic Fairness Verification with Graphical Models is accepted in AAAI 2022. This is a joint work with Debabrota Basu and Kuldeep S. Meel.

I have been invited for a research visit to Inria, Lille Nord, France. I will be working with Debabrota Basu on fairness in Machine Learning.

Our paper on social-spatial group queries with keywords has been accepted in ACM Transactions on Spatial Algorithms and Systems (TSAS). This is a joint work with Sajid Hasan Apon, Mohammed Eunus Ali, and Timos Sellis.

We have improved fairness-verification for linear classifiers both in terms of accuracy and scalability. The paper is available in arXiv now.

I have completed my internship at Goldman Sachs. During my internship, I have experimented with recent advances in transformer-based models, such as BERT, in natural language processing (NLP).

I have joined Goldman Sachs, Singapore as an AI research intern.

Our work on formal fairness verification based on Stochastic Boolean Satisfiability (SSAT) is accepted in AAAI 2021. This is a joint work with Debabrota Basu and Kuldeep S. Meel.

All news....

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Education

National University of Singapore

Jan 2018 - On going

Ph.D. in Computer Science

Bangladesh University of Engineering and Technology

Feb 2013 - Sept 2017

BSc in Computer Science and Engineering