When the number of classes is large, hierarchical classification might be useful. In hierarchical classification, you have a classifier to first classify each example into one of the large groups. Then you have another classifier to classify this example into one of the subgroups. For example, for product classification, you can first classify each product into one of the four main categories: electronics, home and kitchen, fashion, or pet supplies. After a product has been classified into a category, say fashion, you can use another classifier to put this product into one of the subgroups:
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