U.S. housing affordability increased in the fourth quarter of 2015, according to the Housing Opportunity Index (HOI) report released Thursday by the National Association of Home Builders (NAHB) and Wells Fargo.

In the quarter, 63.3% of all homes (including both new and existing) that sold on the market from October 2015 through the end of the year were affordable to families making the median income in the U.S. This is a 110-basis point increase from the 62.2% posted in the third quarter, and is 50 basis points up from the 62.8% recorded a year earlier. It is common that home affordability picks up in winter months, as a result of prices falling in a less active home buying environment.

Median home prices in the U.S. went down to $226,000 from the third quarter’s $231,000, but are still 5.1% up from the $215,000 of the fourth quarter in 2014. This is the fourth time in a row that the fourth quarter median home price has shown year-over-year growth since 2011.

Meanwhile, average mortgage interest rates dropped to 4.09% after climbing up to 4.18% in the third quarter. Plunging oil prices and the shaky global economy pushed mortgage rates further down, despite the fact that the Federal Reserve raised interest rates for the first time in nearly 10 years.

The NAHB quarterly Housing Opportunity Index release is a gauge of the share of affordable housing on the market for families earning the national median income. A total of 225 metropolitan areas were covered in the research for the fourth quarter of 2015.