Dealing with Unbalanced Classes in Machine Learning - deep ideas

In many real-world classification problems, we stumble upon training data with unbalanced classes. This means that the individual classes do not contain the same number of elements. For example, if we want to build an image-based skin cancer detection system using convolutional neural networks, we might encounter a dataset with about 95% negatives and 5% positives. This is for good reasons: Images associated with a negative diagnosis are way more common than images with a positive diagnosis. Rather than regarding this as a flaw in the dataset, we should leverage the additional information that we get.