Adway Mitra

LogUCB: an explore-exploit algorithm for comments recommendation

The highly dynamic nature of online commenting environments makes accurate ratings prediction for new comments challenging. In such a setting, in addition to exploiting comments with high predicted ratings, it is also critical to explore comments with high uncertainty in the predictions. In this paper, we propose a novel upper confidence bound (UCB) algorithm called LOGUCB that balances exploration with exploitation when the average rating of a comment is modeled using logistic regression on its features. At the core of our LOGUCB algorithm lies a novel variance approximation technique for the Bayesian logistic regression model that is used to compute the UCB value for each comment. In experiments with a real-life comments dataset from Yahoo! News, we show that LOGUCB with bag-of-words and topic features outperforms state-of-the-art explore-exploit algorithms.

A Layered Dirichlet Process for Hierarchical Segmentation of Sequential Grouped Data

We address the problem of hierarchical segmentation of sequential grouped data, such as a collection of textual documents, and propose a Bayesian nonparametric approach for this problem. Existing Bayesian nonparametric models such as the sticky HDP-HMM are suitable only for single-layer segmentation. We propose the Layered Dirichlet Process (LaDP), where each layer has a countable set of Dirichlet Processes, draws from which define a distribution over the countable set of Dirichlet Processes at the next layer. Each data item gets assigned to a distribution (index) from each layer of the hierarchy, leading to hierarchical segmentation of the sequence. The complexity of inference depends upon the exchangeability assumptions for the measures at different layers. We propose a new notion of exchangeability called Block Exchangeability, which lies between Markov Exchangeability (used in HDP-HMM) and Complete Group Exchangeability (used in HDP), and allows for faster inference than Markov Exchangeability. Using experiments on a news transcript dataset and a product review dataset, we show that LaDP generalizes better than existing non-parametric models for sequential data, and by simultaneously segmenting at multiple levels, outperforms existing models in terms of single-layer segmentation. We also show empirically that using Block Exchangeability greatly speeds up inference and allows trading off accuracy for execution time.

Bayesian Modeling of Temporal Coherence in Videos for Entity Discovery and Summarization

A video is understood by users in terms of entities present in it. Entity Discovery is the task of building appearance model for each entity (e.g., a person), and finding all its occurrences in the video. We represent a video as a sequence of tracklets, each spanning 10-20 frames, and associated with one entity. We pose Entity Discovery as tracklet clustering, and approach it by leveraging Temporal Coherence (TC): the property that temporally neighboring tracklets are likely to be associated with the same entity. Our major contributions are the first Bayesian nonparametric models for TC at tracklet-level. We extend Chinese Restaurant Process (CRP) to TC-CRP, and further to Temporally Coherent Chinese Restaurant Franchise (TC-CRF) to jointly model entities and temporal segments using mixture components and sparse distributions. For discovering persons in TV serial videos without meta-data like scripts, these methods show considerable improvement over state-of-the-art approaches to tracklet clustering in terms of clustering accuracy, cluster purity and entity coverage. The proposed methods can perform online tracklet clustering on streaming videos unlike existing approaches, and can automatically reject false tracklets. Finally we discuss entity-driven video summarization- where temporal segments of the video are selected based on the discovered entities, to create a semantically meaningful summary.

Bayesian approach to Spatio-temporally Consistent Simulation of Daily Monsoon Rainfall over India

Simulation of rainfall over a region for long time-sequences can be very useful for planning and policy-making, especially in India where the economy is heavily reliant on monsoon rainfall. However, such simulations should be able to preserve known spatial and temporal characteristics of rainfall over India. General Circulation Models (GCMs) are unable to do so, and various rainfall generators designed by hydrologists using stochastic processes like Gaussian Processes are also difficult to apply over the highly diverse landscape of India. In this paper, we explore a series of Bayesian models based on conditional distributions of latent variables that describe weather conditions at specific locations and over the whole country. During parameter estimation from observed data, we use spatio-temporal smoothing using Markov Random Field so that the parameters learnt are spatially and temporally coherent. Also, we use a nonparametric spatial clustering based on Chinese Restaurant Process to identify homogeneous regions, which are utilized by some of the proposed models to improve spatial correlations of the simulated rainfall. The models are able to simulate daily rainfall across India for years, and can also utilize contextual information for conditional simulation. We use two datasets of different spatial resolutions over India, and focus on the period 2000--2015. We consider metrics to study the spatio-temporal properties of the simulations by the models, and compare them with the observed data to evaluate the strengths and weaknesses of the models.

Reconciliation of categorical opinions from multiple sources

Reconciling opinions from multiple sources on questions of interest to determine the correct answers is an important problem encountered in collaborative information systems such as Q & A forums and prediction markets. Our current work focuses on a widely applicable variant of the above problem where the opinions and answers are categorical-valued with the set of values possibly varying across questions. Most of the existing techniques are tailored only for binary opinions and cannot be effectively adapted for questions with categorical opinions. To address this, we propose a generic Bayesian framework for opinion reconciliation that can readily incorporate latent and observed attributes of sources and subjects. For the scenario of interest, we derive three specific model instantiations of the general approach (CTM, CTM-OSF, CTM-LSG), which respectively capture the latent source behavior, variations of source behavior across subject groups, and inter-source correlations. Empirical results on real-world datasets point to the relative superiority of the proposed models over existing baselines.

Eigen-profiles of spatio-temporal fragments for adaptive region-based tracking

We propose a novel space-time descriptor for region-based tracking which is very concise and efficient. The regions represented by covariance matrices within a temporal fragment, are used to estimate this space-time descriptor which we call the Eigenprofiles(EP). EP so obtained is used in estimating the Covariance Matrix of features over spatio-temporal fragments. The Second Order Statistics of spatio-temporal fragments form our target model which can be adapted for variations across the video. The model being concise also allows the use of multiple spatially overlapping fragments to represent the target. We demonstrate good tracking results on very challenging datasets, shot under insufficient illumination conditions.