et al. (2007e).

![Planar maximally filtered graph of the correlation matrix of the considered example. The vertices indicate the stocks. Colors indicate the different economic sectors, red for technology, blue for energy and green for financial. The thickness of links is proportional to the bootstrap percentage and this value is the number close to each link.[]{data-label="pmfg10"}](PMFG_3d_NEW){width="100.00000%"}

The statistical reliability of regons of hierarchical trees and correlation based graphs cannot be theoretically evaluated in spite of the fact that the statistical reliability of the spectral properties of the correlation matrix can be assessed under the assumption of multivariate normal distribution for the time series of the elements of the investigated set. In the absence of such a theoretical approach we have devised a method to evaluate the statistical reliability of nodes in a hierarchical tree obtained by using a correlation matrix as a similarity measure and links in a correlation based graph. The method we use is based on a bootstrap procedure of the time series used to compute the correlation matrix of the system. The method is detailed in Tumminello et al. (2007d) and Tumminello et al. (2007c). Here we just sketch the most important aspects of the procedure allowing 