*gather* algorithm is more adequate for determining hierarchical structure, while the *scatter* method is more adequate for defining the size of the smallest group in a distribution of points on a map.

![image](./f7im.eps){width="6.0in"}

The *gather* algorithm was better suited for the task of detecting and separating sub-structures. We applied *gather* to the distribution of all Class II YSO candidates in the W3 region, which is the most complete of our samples. Densities were calculated with a square kernel of 36$\arcsec$ over a Nyquist sampled RA-DEC grid. In Figure \[fig:gather\] we show the resultant map, in the form of surface density contours overlaid on an infrared emission image that combines data from the Spitzer MIPS 24$\mu$m, 70$\mu$m from Herschel PACS and 250 $\mu$m from Herschel SPIRE. If we compare the density map to the spatial distribution of sources in Figure \[fig:ysopos\] we see how *gather* highlights the three main regions (IC 1795, W3-Main and W3(OH)) as the principal clusters, but also shows a number of peaks that clearly define significant sub-structure in the spatial distribution of young sources.

The positions and equivalent sizes of sub-structure peaks in the *gather* map are listed in Table \[tab:substructures\]. Some of the sub-structure peaks in 