A new study by the Pew Charitable Trusts suggests that the gentrification of Philadelphia neighborhoods may be far less common than assumed.

Only 15 of the city's 372 residential census tracts - 4 percent - gentrified between 2000 and 2014, the analysis found.

Ten times as many tracts experienced significant drops in median income, as the number of residents living in poverty grew by more than 60,000.

"I was a little surprised to see how few of the neighborhoods qualified as gentrified," said Larry Eichel, director of Pew's Philadelphia research initiative.

The study was officially released at 5 p.m. Thursday.

It offers a new view on an issue that generates anger, frustration and examination, particularly among long-time residents who see costs rise and neighbors depart as boutiques, coffee shops, swank restaurants and wealthier outsiders arrive in older neighborhoods.

The term itself means different things to different people. In using an income-based definition, the Pew study said, "we found that gentrification in Philadelphia was a limited phenomenon."

That finding comes with a host of qualifiers, particularly in relying on defining gentrification through changes and levels of income. Studies that found wider gentrification have looked at rising housing costs, increases in education, and other factors.

The Pew study does not include areas that gentrified before 2000, that changed within the last two years, or are changing now. Eichel noted that a number of areas, particularly around the city's universities, experienced changes often associated with gentrification. But changes in income there did not meet the definition set by Pew.

The study, entitled Philadelphia's Changing Neighborhoods: Gentrification and Other Shifts Since 2000, defined gentrified neighborhoods as those that have shifted from a predominantly low-income population to a significantly higher-income one. It broke the types of gentrified neighborhoods into four groups, based on what they looked like in 2000:

-- Predominantly working-class African-American neighborhoods, including three tracts in the Graduate Hospital area where "gentrification was swift and sweeping."

-- Old industrial neighborhoods, including two tracts in Northern Liberties where new development has been "establishing residential markets where none existed."

-- Mixed-income mostly white neighborhoods, which include five tracts in South Philadelphia and one in Roxborough. Increases in median income qualified the tracts as gentrified although "they experienced less change than the other gentrified neighborhoods in terms of housing market conditions, race, and income."

-- Nonaffluent sections of Center City and adjacent areas.

Eichel said the data showed that "gentrification is a relatively small part of the story for the city's changing neighborhoods." But what occurs in areas that have gentrified, or are in the process of doing so, can significantly impact the community, he added.

The 59,747 people living in the gentrified tracts tracts represented 4 percent of the city population in 2014. All the tracts, except for one in Roxborough, were located near or in Center City.

During the study period, the population of those 15 tracts grew about 13 percent, while the population of the city grew by about 2 percent. Most of the population growth in the 15 tracts was due to a rise in the number of white residents, while there was a net loss of black residents.

Among other findings:

-- The gentrified neighborhoods were not among the city's lowest-income areas in 2000.

-- Twelve of the 15 tracts had higher percentages of white residents in 2000 than the city as a whole, and all 15 had larger proportions of whites in 2014.

-- The three predominantly working-class African-American tracts that gentrified, all in the Graduate Hospital area, underwent the most dramatic changes in racial composition. The total black population fell from 7,793 in 2000 to 3,450 in 2014, while the number of white residents tripled.

For many people, gentrification is a dirty word, describing neighborhoods altered by rising housing values, an influx of new, high-income residents, and the departure of poor or working-class residents who can no longer afford to live there. The topic has generated debate in Philadelphia and across the nation as poorer neighborhoods have been remade.

Last year, a study by Governing magazine reached a different conclusion than Pew. It found that 29 percent of Philadelphia's low-income census tracts underwent gentrification between 2000 and 2013. That constituted explosive change, because in the 1990s, Governing said, only 1.5 percent of city census tracts gentrified.

The magazine acknowledged there is no universal definition of gentrification. For instance, its findings relied partly on the percentage of adults with bachelors' degrees living in any particular tract.

A 2015 paper from the Federal Reserve Bank of Philadelphia, which based its results on Census and credit-score data, found that residents do leave gentrifying areas at slightly higher rates. But it's people with higher credit scores who are more likely to leave, heading to suburbs or wealthier parts of the city, the paper said.

What occurs, the paper said, is that improvements in services and living conditions can make a neighborhood more attractive, creating greater incentive for residents to stay.

To be considered gentrified by Pew researchers, a census tract had to meet three conditions:

-- A relatively low median household income in 2000. Pew set the threshold at $53,992, which was 80 percent of the regional median of $67,490 in inflation-adjusted dollars.

-- Median income had to have increased at least 10 percent in inflation-adjusted dollars from 2000 to 2014, a period in which the city's overall median income dropped 10 percent.

-- The tract's 2014 median household income exceeded the citywide median of $37,460.

Pew relied on income, it said, because the figures were more trustworthy over time.