Streaming data is one of the attention receiving sources for concept-evolution studies. When a new class occurs in the data stream it can be considered as a new concept and so the concept-evolution. One attractive problem occurring in the concept-evolution studies is the recurring classes from our previous study. In data streams, a class can disappear and reappear after a while. Existing studies on data stream classification techniques either misclassify the recurring class or falsely identify the recurring classes as novel classes. Because of the misclassification or false novel classification, the error rates increases on those studies. In this paper we address the problem by defining a novel ensemble technique “class-based” ensemble which replaces the traditional “chunk-based” approach in order to detect the recurring classes. We discuss the details of two different approaches in class-based ensemble and explain and compare them in detail. Different than the previous studies in the field, we also prove the superiority of both “class-based” ensemble method over state-of-art techniques via empirical approach on a number of benchmark data sets including Web comments as text mining challenge.

We propose a novel class-based micro-classifier ensemble classification technique (MCE) for classifying data streams. Traditional ensemble-based data stream classification techniques build a classification model from each data chunk and keep an ensemble of such models. Due to the fixed length of the ensemble, when a new model is trained, one existing model is discarded. This creates several problems. First, if a class disappears from the stream and reappears after a long time, it would be misclassified if a majority of the classifiers in the ensemble does not contain any model of that class. Second, discarding a model means discarding the corresponding data chunk completely. However, knowledge obtained from some classes might be still useful and if they are discarded, the overall error rate would increase. To address these problems, we propose an ensemble model where each class information is stored separately. From each data chunk, we train a model for each class of data. We call each such model a micro-classifier. This approach is more robust than existing chunk-based ensembles in handling dynamic changes in the data stream. To the best of our knowledge, this is the first attempt to classify data streams using the class-based ensembles approach. When the number of classes grow in the stream, class-based ensembles may degrade in performance (speed). Hence, we sketch a cloud-based solution of our class-based ensembles to handle a large number of classes effectively. We compare our technique with several state-of-the-art data stream classification techniques on both synthetic and benchmark data streams, and obtain much higher accuracy.

Concept-evolution has recently received a lot of attention in the context of mining data streams. Concept-evolution occurs when a new class evolves in the stream. Although many recent studies address this issue, most of them do not consider the scenario of recurring classes in the stream. A class is called recurring if it appears in the stream, disappears for a while, and then reappears again. Existing data stream classification techniques either misclassify the recurring class instances as another class, or falsely identify the recurring classes as novel. This increases the prediction error of the classifiers, and in some cases causes unnecessary waste in memory and computational resources. In this paper we address the recurring class issue by proposing a novel "class-based" ensemble technique, which substitutes the traditional "chunk-based" ensemble approaches and correctly distinguishes between a recurring class and a novel one. We analytically and experimentally confirm the superiority of our method over state-of-the-art techniques.

Concept-evolution is one of the major challenges in data stream classification, which occurs when a new class evolves in the stream. This problem remains unaddressed by most state-of-the-art techniques. A recurring class is a special case of concept-evolution. This special case takes place when a class appears in the stream, then disappears for a long time, and again appears. Existing data stream classification techniques that address the concept-evolution problem, wrongly detect the recurring classes as novel class. This creates two main problems. First, much resource is wasted in detecting a recurring class as novel class, because novel class detection is much more computationally- and memory-intensive, as compared to simply recognizing an existing class. Second, when a novel class is identified, human experts are involved in collecting and labeling the instances of that class for future modeling. If a recurrent class is reported as novel class, it will be only a waste of human effort to find out whether it is really a novel class. In this paper, we address the recurring issue, and propose a more realistic novel class detection technique, which remembers a class and identifies it as "not novel" when it reappears after a long disappearance. Our approach has shown significant reduction in classification error over state-of-the-art stream classification techniques on several benchmark data streams.