Structural zeros in regression models with compositional explanatory variables

In many real situations, it may happen to consider a regression model with compositional explanatory variables. Compositional data describe parts of some whole, having the feature to sum to a fixed value, so they are commonly presented as vectors of proportions, percentages, or frequencies. In the compositional framework, the presence of structural zeros in the regressors is problematic, since a composition is not allowed to have a part equal to zero. In the recent years, a few tecniques have been introduced in the literature to face this issue. In this paper a description and a comparison of the most interesting proposals are provided.