From the WSJ:
The work by DeutscheDPB.XE +0.22% economists Peter Hooper, Torsten Slok and Matthew Luzzetti ranks different countries’ housing markets according to an average of two ratios: home prices to income and home prices to rent. Those metrics are calculated for each country on the basis of how far or below they are from their historical norms and then combined with equal weights to come up with an overall estimate of over- or undervaluation.