Study that "Proves" the 50% rule

[Shaun Reilly]

In the past there has been reference to a study done over some large number or rental units that showed the expense ratio was close to 50% (want to say something like on average 49.6% or something like that).

I have never seen this report and I was trying to Google it a few different ways but nothing particularly close to what I have in my head was coming up (Mostly articles or reports about how much tenants pay in rent).

Does anyone know where to find this study, if it in fact actually exists? 

I may be wrong but I feel that @Jon Holdman you were one of the people I have seen talk about this in the past.