U.S. household formations are at their lowest since 1947, data from the Census Bureau show. And that’s helping to keep the supply of unsold homes at near-record levels nationwide, even though relatively few houses are being added to the inventory. Between March 2009 and March 2010, the number of households rose just 357,000, according to the census data. In the previous 12 months, the number increased only 398,000, the third-smallest increase on record since World War II. Between 2002 and 2007, before the economy started on its downward trajectory, household formations averaged 1.3 million a year, U.S. census data show.