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.