A data matrix may be seen simply as a means of organizing observations into
rows ( e.g., by measured object) and into columns ( e.g., by measured variable)
so that the observations can be analyzed with mathematical tools. As a
mathematical object, a matrix defines a linear mapping between points
representing weighted combinations of its rows (the row vector space) and
points representing weighted combinations of its columns (the column vector
space). From this perspective, a data matrix defines a relationship between the
information that labels its rows and the information that labels its columns,
and numerical methods are used to analyze this relationship. A first step is to
normalize the data, transforming each observation from scales convenient for
measurement to a common scale, on which addition and multiplication can
meaningfully combine the different observations. For example, z-transformation
rescales every variable to the same scale, standardized variation from an
expected value, but ignores scale differences between measured objects. Here we
develop the concepts and properties of projective decomposition, which applies
the same normalization strategy to both rows and columns by separating the
matrix into row- and column-scaling factors and a scale-normalized matrix. We
show that different scalings of the same scale-normalized matrix form an
equivalence class, and call the scale-normalized, canonical member of the class
its scale-invariant form that preserves all pairwise relative ratios.
Projective decomposition therefore provides a means of normalizing the broad
class of ratio-scale data, in which relative ratios are of primary interest,
onto a common scale without altering the ratios of interest, and simultaneously
accounting for scale effects for both organizations of the matrix values. Both
of these properties distinguish it from z-transformation.