Between that oil crash and the S&L debacle, the real estate market here was decimated unlike any other period in my lifetime. 

Duplexes in my neighborhood get pretty much the same rental yield as a SFR assuming you were to buy one now (5% with a varian of a few decimals), if your neighborhood is like mine, you might as well just rent it out as a SFR if you are getting the same yield and maintain the liquidity of your property, as I generally find duplexes a little harder to sell compared to a SFR in my area.

He will no longer have his daily cashflow decimated due to daily debits, which can sometimes lead to overdrafts.Business owner will be able to get his credit back in shape.Business owner will exercise his right to re-purchase his home as he'll be qualified for a HELOC / 2nd mortgage.He will have learned his lesson, I will have made a nice return on my capital while holding the deed to the property where he owns 50% equity as my security. 

Also, the REI returns could be decimated by sell costs ( 10% to agent+closing costs  and 25% recapture tax)Only experienced investors should go 100% stocks.

Cheap natural gas will decimate coal use over some period.Have you noticed how fast LED bulbs have replaced small incandecent bulbs?  

Since we are talking vacancy here, I actually subtract that number from 365 to get unpaid vacancy days per unit per year.This can then be expressed as a a percentage: # of vacancy days / 365 (decimal) X 100 (to get %).This is the vacancy rate per unit.

A Multifamily in Philadelphia cash flowing like gang buster can be decimated by property taxes if placed in New Jersey.

Response rate is about .5% (not the decimal in front of the "5").

Or a major employer pulls out of town and his tenant pool is decimated.

I created a few basic formulas to calculate certain values and ratios, but for the most part, it's really about how many decimal places you have in the account and keeping up with each dollar as it goes in and out.