He tells me:
WalMart wages were very difficult to get, but we found nothing to indicate that WalMart adjusted wages by cost of living. So if the article did not make it clear, the comparison is the wage gap if WalMart entered the Bay Area paying their nationally prevailing wage. We focused on benefits because a lot of the compensation gap was benefits, and assuming WalMart would enter Bay Area with their national prevailing benefits policy seemed even more reasonable.
On my re-read, we handled [whether comparing Wal-mart wages to SF wages was ok] on p. 441 and footnote 10.
The part that is amazing to me now, and was evident then, is that half the gap was benefits. I wonder how much of this issue is attenuated with Obamacare?