use this observational definition of OC (see also Section \[sec:classification-oc-ec\]) in order to account for those objects that observationally appear like classical OCs, but whose dynamical state is unknown, in some cases they can actually be gravitationally unbound.

-   a *physical OC* as a gravitationally bound OC (i.e., a classical OC).

-   an *association* as an unbound OC.

In this work, we sometimes use the term “star clusters” generically for all the classes defined above, especially when concerning observations. Bound, exposed star clusters, however, will be always be referred to explicitly as *physical OCs*.

Compilation of cluster lists {#sec:catalogs-summary}
============================

Although the number of known OCs and ECs in the Galaxy has considerably increased over the last years, the current cluster sample is still far from being complete. As we discuss in Section \[sec:completeness\], the detection of a stellar cluster in the inner Galactic plane is particularly difficult, due to the high extinction and the crowded stellar background, making the cluster sample severely incomplete for distances larger than a few kpc from the Sun. If we are able to quantify this incompleteness, however, all the statistical results can properly be corrected, as we do in this work. Of course, the 