The most popular methods for finding a linear transform as in Eq. (2) are second-order methods. This means methods that find the representation using only the information contained in the covariance matrix of the data vector . Of course, the mean is also used in the initial centering. The use of second-order techniques is to be understood in the context of the classical assumption of Gaussianity. If the variable has a normal, or Gaussian distribution, its distribution is completely determined by this second-order information. Thus it is useless to include any other information. Another reason for the popularity of the second-order methods is that they are computationally simple, often requiring only classical matrix manipulations.
The two classical second-order methods are principal component analysis and factor analysis, see [51,77,87]. One might roughly characterize the second-order methods by saying that their purpose is to find a faithful representation of the data, in the sense of reconstruction (mean-square) error. This is in contrast to most higher-order methods (see next Section) which try to find a meaningful representation. Of course, meaningfulness is a task-dependent property, but these higher-order methods seem to be able to find meaningful representations in a wide variety of applications [36,46,80,81].