Minimal Spanning Tree Based Tests

This method is inspired by the Wald-Wolfowitz, and solves the problem that there is no natural multidimensional `ordering'.

The main reference is Friedman and Rafsky 1979, Ann stat.,pp. 697-717.

One may either use multidimensional data or first make the data bidimensionnal by principal components. (or another dimension reducing technique), in order to be able to represent the tree in a graphic.