Strauss said AVI, which Nutter undertook to fix the city's widely inaccurate list of property values, is "a story about well-intentioned policy change but not effective implementation."
On average, he said, there is a 112 percent difference between the true market value of residential properties and the administration's new assessments. The current assessments vary by 82 percent on average, Strauss said.
The Office of Property Assessment said its own review found the new numbers to vary by only 13.9 percent, which is within the industry standard of 15 percent.
Nutter spokesman Mark McDonald said Strauss did not consult the Property Assessment Office during his study.
"The author could not have known the complicated set of assumptions and decisions that went into the process," McDonald said.
Butkovitz's findings, McDonald said, "have to be seen for what they are - a $27,000 report that comes out a few weeks before a city controller runs for re-election."
Butkovitz faces Brett Mandel and Mark Zecca in the May 21 Democratic primary.
On PhillyDailyNews.com: Need more information on the city's Actual Value Initiative? Get it on our AVI topic page.
On Twitter: @SeanWalshDN