The HOI data released in the third quarter of 2019 incorporates the use of Freddie Mac's 30-year fixed effective rate series, following the discontinuation in mid-2019 of the FHFA series previously used in the HOI calculations. The new, updated HOI series goes back to the first quarter of 2012.
The Housing Opportunity Index (HOI) for a given area is defined as the share of homes sold in that area that would have been affordable to a family earning the local median income, based on standard mortgage underwriting criteria. Therefore, there are really two major components -- income and housing cost.
For income, NAHB uses the annual median family income estimates for metropolitan areas published by the Department of Housing and Urban Development. NAHB assumes that a family can afford to spend 28 percent of its gross income on housing; this is a conventional assumption in the lending industry. That share of median income is then divided by twelve to arrive at a monthly figure.
On the cost side, NAHB receives every month a CD of sales transaction records from CoreLogic. The data include information on state, county, date of sale, and sales price of homes sold. The monthly principal and interest that an owner would pay is based on the assumption of a 30 year fixed rate mortgage, with a loan for 90 percent of the sales price (i.e., 10 percent downpayment). The interest rate is an average of the 30-year fixed effective rate from Freddie Mac's Primary Mortgage Market Survey during that quarter. In addition to principal and interest, cost also includes estimated property taxes and property insurance for that home. This is based on metropolitan estimates of tax and insurance rates from the the most recent American Community Survey. Mortgage insurance is not currently a component of the HOI.
Therefore, for each record, there is an estimated monthly cost and available income share. The HOI is the share of records in a metropolitan area for which the monthly income available for housing is at or above the monthly cost for that unit.