Councilman Mark Squilla, whose district faces some of the starkest jumps in value, complained last week of large errors. At-large Councilman James F. Kenney - who is considering a run for mayor in 2015 - promised a "militant" response to the new values.
Values could be different from a recent sales price for a number of reasons - for one, the buyer might have overpaid or gotten a deal, McKeithen said.
"The sale price of your house does not necessarily dictate what your assessment will be," he said.
Or the property might have been a boarded-up shell at sale but was rehabbed by the time it was assessed.
Or the assessor might have been flat wrong, in which case McKeithen asked property owners to file a first-level review with the Office of Property Assessment (OPA).
By industry standards, McKeithen said, the average difference between the sale price and assessment should be no more than 15 percent. The gap in the city before the reassessment was 27 percent, he said.
McKeithen said OPA came under that goal, hitting 13.9 percent.
That still means a house that would sell for $300,000 could have a fairly wide range in assessment, anywhere from $258,300 to $341,700.
At a theoretical tax rate of 1.25 percent, two houses selling for $300,000 with such disparate assessments would also have widely different tax bills - $3,229 or $4,271. City Council will set the tax rate as part of the budget process, to be finished by June 30.
Not all properties will fall within that 15 percent range of a recent sales price, but that shouldn't cast doubt on the overall effort, said Kevin Gillen, an economist at the University of Pennsylvania who worked as a consultant for the OPA on the reassessment, known as the Actual Value Initiative.
Identifying "anecdotal properties" outside the 15 percent range as a basis for attacking the assessment effort's credibility "is extremely misleading and outright erroneous," Gillen said.
But a larger sampling raises questions. An Inquirer analysis of about 10,000 residential sales from 2012 showed only 40 percent of the homes would be assessed within 15 percent of the sale price. The Inquirer excluded sales below $10,000 and other transactions, including sheriff's sales and those between related parties.
The OPA determined its 13.9 percent benchmark based on five years of sales, minus sales that were not arm's-length transactions between unrelated parties, as well as sales that were deemed poor indicators of a similar home's worth.
Sales data from each year also were weighted to account for fluctuations in the market.
Gillen noted that state law prohibits the city from assessing a house solely on its sale price - the OPA is required to consider the sales of neighboring properties.
Those gaps in value mean real dollars to real taxpayers. But, McKeithen said, this is a starting point; the assessments will improve each year, as more data are compiled about sales and the shifting marketplace.
So how did assessors come up with the values?
The OPA started the process by carving up the city into 14 large zones and, using sales and other data, determining a base property.
"We define a typical property. . . . It has a set of characteristics," said Kevin Keene, the OPA's real-property assistant administrator. "Then, for your property, we say how are you different from the base property."
Central air-conditioning could add value. A garage would tack on value - more in areas with little parking. Living near a park or in the catchment of a great school could add big money.
On the negative side, living next to a busy high school, with thousands of students coming and going, depresses worth. Even having a bigger house could be a drawback in a neighborhood of seniors looking to downsize.
The OPA, however, did not take into account amenities for which good data were not available - and that included factors such as how many bathrooms are in a house.
Assessors further broke the city down into Geographic Market Areas (GMA), which also reflect neighborhoods considered more valuable or less desirable than the norm.
Being grouped in one GMA or another could boost a property's value or drag it down. Some stark dividing lines can be seen between the GMAs - such as in a section of West Philadelphia.
In the 4900 block of Walton Avenue, in a GMA in western University City, houses are valued at about $235,000. Across 50th Street, within a GMA tilting west toward Cobbs Creek, similar homes are valued at about $102,000.
Neighbors on both sides of the street said their homes were probably worth about $200,000.
Paula Moore, a retired teacher's aide, lives on the east side of 50th. Her bill is likely to increase by more than $2,100 - from about $800 to more than $2,900, like most on her block. That's an amount she's afraid she can't afford.
"In 13 years, I've watched this block change," she said. "This is what they want to do, is force people out."
On the west side of 50th, Roger McLeod and his wife expect their bill to go up by just $126. Although he thinks the city assessed his house at much less than he could sell it for, he recognizes that his neighbors did not fare as well.
"That still don't add up," he said.
McKeithen said the GMAs would be refined in future years as more data are collected.
"We're happy with where it turned out for our first year," he said. "We know we still have some work to do."
Contact Troy Graham
at 215-854-2730, email@example.com, or follow on Twitter @troyjgraham.
Inquirer staff writers Harold
Brubaker and Miriam Hill contributed to this article.