A U.S. housing affordability index compiled by the National Association of Realtors reached a record high in the first quarter. NAR's affordability index gauges whether a median-income family could qualify for a conventional mortgage loan on a median-priced, existing single-family home. The index takes into account mortgage interest rates and assumes a 20 percent down payment and a monthly principal and interest payment not exceeding 25 percent of the gross median family monthly income.An index value of 100 means that a median-income family has exactly enough income to qualify for a mortgage covering 80 percent of the cost of a median-priced home, while an index above 100 means that family has more than enough income to qualify. In the first quarter, the quarterly composite index posted a ...