Continual learning from data streams is among the most important topics in contemporary machine learning. One of the biggest challenges in this domain lies in creating algorithms that can continuously adapt to arriving data. However, previously learned knowledge may become outdated, as streams evolve over time. This phenomenon is known as concept drift and must be detected to facilitate efficient adaptation of the learning model. While there exists a plethora of drift detectors, all of them assume that we are dealing with roughly balanced classes. In the case of imbalanced data streams, those detectors will be biased towards the majority classes, ignoring changes happening in the minority ones. Furthermore, class imbalance may evolve over time and classes may change their roles (majority becoming minority and vice versa). This is especially challenging in the multi-class setting, where relationships among classes become complex. In this paper, we propose a detailed taxonomy of challenges posed by concept drift in multi-class imbalanced data streams, as well as a novel trainable concept drift detector based on Restricted Boltzmann Machine. It is capable of monitoring multiple classes at once and using reconstruction error to detect changes in each of them independently. Our detector utilizes a skew-insensitive loss function that allows it to handle multiple imbalanced distributions. Due to its trainable nature, it is capable of following changes in a stream and evolving class roles, as well as it can deal with local concept drift occurring in minority classes. Extensive experimental study on multi-class drifting data streams, enriched with a detailed analysis of the impact of local drifts and changing imbalance ratios, confirms the high efficacy of our approach.

Learning from imbalanced data and data stream mining are among most popular areas in contemporary machine learning. There is a strong interplay between these domains, as data streams are frequently characterized by skewed distributions. However, most of existing works focus on binary problems, omitting significantly more challenging multi-class imbalanced data. In this paper, we propose a novel framework for learning from multi-class imbalanced data streams that simultaneously tackles three major problems in this area: (i) changing imbalance ratios among multiple classes; (ii) concept drift; and (iii) limited access to ground truth. We use active learning combined with streaming-based oversampling that uses both information about current class ratios and classifier errors on each class to create new instances in a meaningful way. Conducted experimental study shows that our single-classifier framework is capable of outperforming state-of-the-art ensembles dedicated to multi-class imbalanced data streams in both fully supervised and sparsely labeled learning scenarios.

Learning from data streams is one of the most promising and challenging domains in modern machine learning. Proliferating online data sources provide us access to real-time knowledge we have never had before. At the same time, new obstacles emerge and we have to overcome them in order to fully and effectively utilize the potential of the data. Prohibitive time and memory constraints or non-stationary distributions are only some of the problems. When dealing with classification tasks, one has to remember that effective adaptation has to be achieved on weak foundations of partially labeled and often imbalanced data. In our work, we propose an online framework for binary classification, that aims to handle the complex problem of working with dynamic, sparsely labeled and imbalanced streams. The main part of it is a novel active learning strategy (MD-OAL) that is able to prioritize labeling of minority instances and, as a result, improve the balance of the learning process. We combine the strategy with a dynamic ensemble of base learners that can abstain from making decisions, if they are very uncertain. We adjust the abstaining mechanism in favor of minority instances, providing an effective method for handling remaining imbalance and a concept drift simultaneously. The conducted evaluation shows that in the challenging and realistic scenarios our framework outperforms state-of-the-art algorithms, providing higher resilience to the combined effect of limited labeling and imbalance.

Data stream mining is among the most contemporary branches of machine learning. The potentially infinite sources give us many opportunities and at the same time pose new challenges. To properly handle streaming data we need to improve our well-established methods, so they can work with dynamic data and under strict constraints. Supervised streaming machine learning algorithms require a certain number of labeled instances in order to stay up-to-date. Since high budgets dedicated for this purpose are usually infeasible, we have to limit the supervision as much as we can. One possible approach is to trigger labeling, only if a change is explicitly indicated by a detector. While there are several supervised algorithms dedicated for this purpose, the more practical unsupervised ones are still lacking a proper attention. In this paper, we propose a novel unsupervised ensemble drift detector that recognizes local changes in feature subspaces (EDFS) without additional supervision, using specialized committees of incremental Kolmogorov-Smirnov tests. We combine it with an adaptive classifier and update it, only if the drift detector signalizes a change. Conducted experiments show that our framework is able to efficiently adapt to various concept drifts and outperform other unsupervised algorithms.

Data stream mining is a rapidly developing branch of contemporary machine learning. Ensemble approaches have proven themselves to be highly effective in this domain, due to their predictive power and capabilities for handling evolving data. One of the key aspects of ensemble learning is diversity among base classifiers - it improves accuracy and allows for anticipating and recovering from concept drifts. It has been shown that while diversity is desirable during changes, it may impede learning when data becomes stationary. In this paper, we present a novel ensemble technique that exploits the idea of dynamic diversification, which increases diversity during changes and reduces it when a stream becomes stable. The algorithm uses online clustering for this task by creating locally specialized base learners trained on spatially related instances. Three control strategies based on the novel range heuristic for managing a trade-off between error (a change indicator) and diversity are utilized. Additionally, two intensification strategies are proposed for exploitation of newly arriving instances, allowing for faster adaptation. Experimental study evaluates the general performance and diversity of the proposed algorithm, proving its capabilities to outperform state-of-the-art ensembles dedicated to drifting data stream mining.