Its just a "rule of thumb" that has been borne out by empirical data.

Hell, there is this burgeoning website, census.gov, which provides essentially any element of demographic data one could ever imagine down to census tract block groups (which, in cities, is a very small, concentrated area). 

Thomas,When you only pull a report from Trans Union's My Smart Move, which is the weakest of all three bureaus and there is a good chance you will miss something because Experian and Equifax will collect data trans union don't collect.My 2 cents,Joe Gore

I have not been in the business long enough to have a lot of personal data, but my understanding of the 50% rule is that it is based on a LOT of long term data.

Next year you may decide to use one and then pay more for him to go back and recreate or review your 2013 data instead of just having it set up correctly from the beginning.

Check with your local housing authority about a pet policy, there should be tons of data and guidelines to pull from and adopt, they have already been there done that.We had cat owners that had cats heavier than 20 pounds!

Is there a database that the data needs to be fed to?

When we first implemented our system, we met a fair amount of confusion and resistance.  

When I was on vacation in Hawaii I met a man from Georgia who rents out to and loves section 8.

It shares some interesting statistical data that I think is relative to my concern.