Clustering reduces the amount of data items by grouping them. There also exist methods that can be used for reducing the dimensionality of the data items, too. Such methods will be called projection methods below. The goal of the projection is to represent the input data items in a lower-dimensional space in such a way that certain properties of the structure of the data set are preserved as faithfully as possible. The projection can be used to visualize the data set if a sufficiently small output dimensionality is chosen.