There's no question that affordability is a key challenge for the housing sector, as it is for the millions of Americans searching for a place to live that is within their budget. However, determining what "affordability" means in the context of a particular city or region and what factors influence it presents a challenge of its own.

At a time when the issue is becoming ever more urgent, a new first-of-its-kind study by Yardi Matrix attempts to take on this challenge. It was guided by the hope that its findings and the transparency of its database would provide insight into the workings of the market and help it operate more efficiently to create affordable housing.

The study defines fully affordable housing as properties in which at least 90% of the units have income restrictions. It determined the fully affordable maximum allowable rent using a per-unit calculation based on HUD income limits, family size adjustments, utility allowances, and "the specific property income restriction splits (how many units' rents are restricted to 40%, 60% or 80% of area median income)."