the principal clusters as consistent samples to study the history of star formation in the complex. The GMM analysis provides such consistent identification, which may differ with respect to other identification methods [e.g.  @Rivera-Ingraham:2011yq].

### Cluster Substructure in W3. Nearest Neighbors. \[s:analysis:ss:ysodist:sss:gather\]

Estimating the surface density of young stellar sources is often done by applying a variant of the nearest neighbor method [formalized for star cluster detection by  @Casertano:1985uq]. This method has been applied successfully in different studies [e.g.  @Gutermuth:2010aa; @Roman-Zuniga:2008aa]. However, two very important issues regarding this method must be pointed out. First, when calculating distances to the $n$th neighbor in a distribution of points on a plane, there is a minimum value $n=6$ to avoid biasing the detection toward small structures formed purely by chance in a 2-dimensional distribution of points. Second, when mapping the surface density defined by such distances, the choice of an optimal smoothing kernel that would not dilute genuine substructures neither generate spurious ones, is not straightforward. These two issues were taken into consideration by [@Gladwin:1999id], who developed two nearest neighbor mapping methods, *gather* and *scatter*, both optimized to correctly detect small groups (in our case, possible cluster substructures) in source position maps. The 