A report commissioned by Philadelphia City Controller Alan Butkovitz says the reassessment that is key to Mayor Nutter's property tax reform effort is largely inaccurate and unfair - even more so than the broken system it was meant to replace.

Butkovitz said Wednesday that "if it were up to me," he would scrap the Actual Value Initiative (AVI) for a second year in a row, and urged Nutter to "invest heavily in getting the assessment right."

"When you're trying to do something this important with such ramifications, there's no excuse for not doing what is required to get it right," he said.

The report, commissioned for $27,500, was written by Robert Strauss, an economics and public policy professor at Carnegie Mellon University in Pittsburgh with experience in Allegheny County's controversial property reassessment. His analysis was based on publicly available data from the city's Office of Property Assessment (OPA).

Mark McDonald, a spokesman for Mayor Nutter, said OPA "conducted the most professional and accurate assessment of property in the city's history." He also said OPA officials offered to meet with Strauss and "never heard back."

Strauss said his study found the average difference between residential sale prices and the new assessments to be as high as 112 percent - and off by as much as eight times in some areas of the city.

The industry standard is a 15 percent difference, and OPA has said the reassessment came under that mark at 13.9 percent.

"We kept trying to find the 14 percent," Strauss said. "We're so far beyond that, in multiples, we couldn't find it, and it wasn't for a lack of trying."

McDonald noted that Butkovitz, facing a contested primary election later this month, long has been one of the most vocal critics of AVI.

"What we have here is a city controller who chose to attack this vastly improved process many months ago and who then hires an 'expert' to support his position," McDonald said.

There are several differences in how Strauss and OPA calculated their results.

OPA judged the reassessment by comparing values to about 20,000 sales over the last five years deemed to be good indicators of the market. Strauss used about a third more sales, all of which OPA described as arm's-length transactions between unrelated parties that were scrubbed from OPA's sample for other reasons.

OPA also weighted sales based on changes in the market over the five years. Strauss didn't do that, but in some studies he adjusted for inflation.

Strauss' property roll also was missing about 20,000 parcels that had not been assessed in March, when he collected the publicly available data. There are about 579,000 parcels.

Strauss said he would not characterize OPA's process, but suggested the city might have been "filtering the data to get to a desired result."

"If you keep reducing the sample because you want to stay within 14 percent, you chase the data, then you're not doing everybody else a service," he said. "We fooled around with the data . . . and the numbers don't change."

McDonald questioned Strauss' results given that he didn't have the complete data set and "didn't follow the procedures used by OPA to look at sales data."

"The result is pretty clear - garbage in, garbage out," he said.

While Butkovitz suggested delaying AVI, doing so would require the state's permission, and there has been no movement in Harrisburg on that front. Instead, the Philadelphia delegation has been focused on legislation to soften AVI's impact.

In Council, where AVI critics are readily found, just one of the 17 members has advocated delaying the reform again. A recent report compiled by Council staff also found that with the right mixture of tax breaks, as many as 72 percent of homeowners could see lower bills and just 10 percent would see their taxes rise by more than $400.

Strauss is considered one of the state's experts on property taxes. He has been a frequent critic and commentator in Allegheny County's long saga to conduct an acceptable reassessment. Last year he created a website for county taxpayers to estimate their bills for 2013 and 2014.

He said many of the issues he found with Philadelphia's reassessment could be attributed to bad and missing data, saying OPA might have been short on time, money, and training to collect the necessary information.

For example, Strauss said, 30 percent of residential properties do not list the number of stories and 26 percent do not list the total number of rooms.

"This is what I found and I stand by it," he said, adding later, "This can't be swept under the rug.