A separation is defined here as the dissolution of a cohabitation relationship by a move of one or both partners. A couple is considered separated if the partners no longer live together at the same address and have not moved back in together again by the end of the following year. Relationship dissolutions as a result of emigration or the death of a partner are not included in this research. When partners do not live together anymore, the separation has not yet been finalised. One can even maintain a LAT relationship (either married or unmarried). On the other hand, it can also happen that the divorce has already been pronounced, while the divorced parents continue to live at the same address for some time.

For these reasons, the figures on (divorce) separations and the children involved differ from the figures in the National Youth Monitor. The National Youth Monitor is based on the registrations of divorce decrees in the divorce register of the municipality where the marriage took place in a given year.