Refers to variables representing fractions of a total, i.e. all lying in $[0,1]$ interval and necessarily summing to one. Analysis of such data is often called compositional data analysis.
Compositional data pertain to the relative proportions of a whole. For example,
- Each data point may correspond to a rock composed of three different minerals; a rock of which 10% is the first mineral, 30% is the second, and the remaining 60% is the third would correspond to the triple [0.1, 0.3, 0.6]; a data set would contain one such triple for each rock in a sample of rocks.
(Wikipedia)
This is subtly different from binomial or multinomial data, which can also yield proportions that lie exclusively in $[0, 1]$ (or could be represented as counts out of a total), but where the proportions come from discrete events that could have been one category or another.
The analysis of such data requires special methods, mostly based on log ratios.
Additional resources can be found at http://www.compositionaldata.com/
