The Inquirer, Wharton School researcher Kevin Gillen, and economist Robert Strauss of Carnegie Mellon University used slightly different methods to arrive at similar results.

All three relied on public data, containing sales and assessment records for more than 570,000 properties, supplied by the Philadelphia Board of Revision of Taxes.

Here is a summary of the methods:

The Inquirer matched more than 25,000 sales prices in 2007 against the latest assessments. Through the use of BRT codes, the sample was limited to "arm's-length transactions" - what willing sellers and buyers would pay - by eliminating such elements as nominal sales and those for which assessments were extremely high or low. The analysis included residential, commercial and industrial properties.

The Inquirer calculated that properties were assessed, on average, at 12.2 percent of value.

Wharton's analysis covered all 2007 residential sales of $10,000 or greater. Gillen eliminated all condominium and apartment properties and filtered the sample for arm's-length transactions of single-family homes, a total of 16,890 sales.

He found that residences were assessed, on average, at 10.1 percent of value.

Strauss studied residential assessments from 2005 to 2007, a common practice in sales-ratio studies. He limited his sample to sales of $500 or more and examined 77,407 records.

He computed a median assessment of 10.9 percent for residences.