Hierarchical star formation leads to a progressive decrease in the clustering of star clusters both in terms of spatial scale and age .
Consistently , the statistical analysis of positions and ages of clusters in the Milky Way disk strongly suggests that a correlation between the duration of star formation in a region and its size does exist .
The average age difference between pairs of open clusters increases with their separation as the \sim 0.16 power .
In contrast and for the Large Magellanic Cloud , Efremov & Elmegreen ( 1998 ) found that the age difference scales with the \sim 0.35 power of the region size .
This discrepancy may be tentatively interpreted as an argument in support of intrinsically shorter ( faster ) star formation time-scales in smaller galaxies .
However , if both the effects of cluster dissolution and incompleteness are taken into consideration , the average age difference between cluster pairs in the Galaxy increases with their separation as the \sim 0.4 power .
This result implies that the characteristic time-scale for coherent , clustered-mode star formation is nearly 1 Myr .
Therefore , the overall consequence of ignoring the effect of cluster dissolution is to overestimate the star formation time-scale .
On the other hand , in the Galactic disk and for young clusters separated by less than three times the characteristic cluster tidal radius ( 10 pc ) , the average age difference is 16 Myr , which suggests common origin .
A close pair classification scheme is introduced and a list of 11 binary cluster candidates with physical separation less than 30 pc is compiled .
Two of these pairs are likely primordial : ASCC 18/ASCC 21 and NGC 3293/NGC 3324 .
A triple cluster candidate in a highly hierarchical configuration is also identified : NGC 1981/NGC 1976/Collinder 70 in Orion .
We find that binary cluster candidates seem to show a tendency to have components of different size ; an evidence for dynamical interaction .