Trulia, the real-estate search engine, has been doing some interesting research - most recently, on the rise in income inequality in the nation's 100 largest metropolitan areas.

To compile a list of the most unequal metro areas, Trulia used U.S. Census data on household income - which I also use, to determine median income for my weekly "Town by Town" columns in Sunday Business - in addition to its own statistics on housing affordability.

What Trulia said about this region should come as no surprise: Philadelphia, poorest of the 10 largest U.S. metro areas, is among the country's most affordable areas for housing.

How does Trulia define rich and poor households?

A rich household is defined as being at the 90th percentile - that is, being above 90 percent of all households in the metro area. The median is at the 50th percentile, while poor is defined as being at the 10th percentile.

Jed Kolko, Trulia's chief economist, said its main inequality measure is the ratio of incomes at the 90th and 10th percentiles (the 90/10 ratio), which shows the size of the gap between rich and poor.

A higher ratio value means incomes are more unequal. Among the 100 metro areas, the 90/10 ratio ranges from below 9 to above 18.

Philadelphia's is 14.7, Trulia said.

By comparison, the gap between the rich and the poor is smallest in Allentown, Kolko said: 9.0.

Inequality has increased in 94 of the 100 largest metro areas between 1990 and 2012, he said.

The increase has accelerated in the last few years, Kolko said. "While the poor are falling farther behind in most metro areas, the rich have pulled farther ahead of the median in all 100 major metros."

The Federal Reserve Bank of Philadelphia reinforces Trulia's observations that the poor are getting poorer. Its fourth-quarter indicators suggest that conditions affecting low- and moderate-income households continued to decline in the final months of 2013, and its job-availability index fell to the lowest level in two-plus years.

The Fed sends questionnaires each quarter to groups in its Third District that provide services to lower-income residents. Fifty-one percent of the respondents provide housing services, the Fed said.

Many organizations that deal with low- and moderate-income Philadelphians expressed concern about their continued loss of funding, a lack of livable-wage jobs, and an even steeper rise in demand for their services after the expiration of emergency unemployment-compensation benefits, the Fed said.

A sample of comments, provided by the Fed without identifying the sources, is indicative of the problems being faced.

Said one organization: "Safe, orderly, and attractive housing and neighborhoods make a huge difference in the lives of families and especially their children. The stagnation of incomes at the lower end of the spectrum has put unsubsidized housing of that sort out of reach for the large majority."

Said another: "The key element to improving the economic situation of low- and moderate-income households is the availability of livable-wage jobs."

And another: "There seems to be a second wave of families, many of whom are ineligible for services, who are trying to find help to stabilize their finances. These are families that had good, stable, fair-wage jobs, but who have lost those positions and are now unemployed or are earning half of what they earned in the past and they can't keep up with their bills."

In general, Trulia's Kolko noted, "Where housing is less affordable, income inequality is more extreme."