This measure is a proxy for the cohabitation rate. It is the number of adults age 18 to 64 who are either an unmarried partner to the household’s reference person or who are reference persons with unmarried partners, as a percentage of the total population age 18 to 64. Cohabiting couples in which neither partner is the reference person are, therefore, not counted as cohabiting. A household’s reference person is the person (or one of the people) who own or rent the housing unit in which a household resides; or, if there is no such person, any adult member, excluding roomers, boarders or paid employees. If the house is owned or rented jointly by a married couple, the reference person may be either the husband or the wife.

A variable that can be used to estimate the cohabitation rate in a direct way has been available in the Current Population Survey since 2007, but the proxy measure allows for the construction of a longer-term trend. From 2007 to the present, changes in the proxy measure have closely tracked changes in the actual cohabitation rate; however, the level of the proxy measures has been consistently lower, by about 1 to 1.5 percentage points.