  Compositional data analysis is concerned with multivariate data that have a
constant sum, usually 1 or 100\%. These are data often found in biochemistry
and geochemistry, but also in the social sciences, when relative values are of
interest rather than the raw values. Recent applications are in the area of
very high-dimensional "omics" data. Logratios are frequently used for this type
of data, i.e. the logarithms of ratios of the components of the data vectors.
These ratios raise interesting issues in matrix-vector representation,
computation and interpretation, which will be dealt with in this chapter.
