Until now, we have always considered training as learning a function that maps some data to some labels. For the tasks in this chapter, we may not possess labels that we can use to learn the classification model. This could be, for example, because they were too expensive to collect. Just imagine the cost if the only way to obtain millions of labels was to ask humans to annotate those manually. What could we do in that case?
We find some pattern within the data itself. This is what we will do in this chapter, where we again consider the challenge of a question and answer website. When a user is browsing our site, perhaps because they are searching for specific information, the search engine will most likely point them to a specific answer. If the presented answers are not what they are looking for, the website should present (at least...