You do not have to become a believer in modern portfolio theory (I am not) but you are not educated if you don’t know it exists and how influential it has become - its tenets (not tenants) literally percolate through much of what you see written in the popular press about investing, retirement strategies, diversification, etc.Statistics - A one semester or one quarter course in statistics will serve you very well all your life, and open your eyes to a deeper understanding of probability and risk.Accounting - You really do need to have a conceptual understanding of what balance sheets, cash flow statements and profit and loss means.

Paper losses are great for deferring taxes.

But so far, these "flips" are just paying me about 20 bucks an hour for my time, and I don't have tons of cash laying around to eat a 600 dollar loss because Lowes sold me a broken refrigerator and doesn't back it up.  

Any place you invest you need to have one of the following be true in my opinion:1) purchase considerably below market value (very challenging in a red hot market)2) force appreciation though increase rents or rehab (easier if you have skills/desire/patience)3) be able/willing to wait for the next cycle (5, 7,10,15, 30 years, think Texas in the late 80s)4) have great positive cash flow that you can weather rent loss or massive reductions in rent to the tune of 30-50%.

I used a 20% loss on gross (10% vacancy, 5% CapEx and 5% repair) on each house. 

At a glance I would tell you to do things in this order before purchasing more properties:1) Build enough cash reserves to cover large emergency expenses on your current properties say $5k-$20k depending on age of the property, value of the property, and your risk tolerance.2) Build a personal emergency fund that will cover you for 3 to 6 months (depending on your risk tolerance again) if you suffer a financial set back (i.e. loss of job, medical problem, etc.)3) Pay off all other debt besides loans on investment properties.4) Make sure you are setting some assets aside for retirement outside of real estate.If you follow this basic formula you are setting yourself up for success no matter what life and the markets throw at you.

Until then, a little concern is in order, but not worth any nightmares or sleep loss yet @Michael Gregory :)

This morning they came and took their ladders and tools only and left all debris so when I took my lunch break today I went outside to remove the debris because I was scared my 4 yo son would venture outside and step on something or have something fall on him and in the process I stepped on a huge nail that went right through my shoe and now I am leaking blood and in terrible pain and still had to return to work so I don't have loss of wages.. 

A non private lender can also be a hard money or asset based lender.While I am sure that the success stories of using private lenders reported on BP are correct, I have been told of many losses from people who made private loans.  

I know several student housing investors, though most are out of the market now, but they assume the worst and mitigate by signing the parents to the leases, charging large deposits, and working the turn losses into their models.