As doctors, we most likely fall into the first category. We may be rich in the sense that our salaries are in the higher economic range, however, because of our expenses (houses, cars, student loan debt, private school tuition, practice overhead, etc.) and sometimes poor decisions, we have a tough time accumulating any real wealth. We’re also handicapped by the fact that we start along this financial journey relatively later in life.
Case Schiller only tracks price appreciation of RE. RE as rental investment vehicle is measured primarily on rental yield or cap rate or some other measure. Price appreciation in that scenario is only a secondary means of growth, and arguably should be ignored as a predictor of returns when deciding on whether or not to invest in rentals. More important key performance indicators for rentals are net operating income and cash ROI. Appreciation, if it occurs, is a bonus.

No one should turn down wind farming’s ultimate passive income for the next 30 or more years … even 60 years when there is a positive cash flow on the sum total of all base payments when computing inflation for the next 60 years based on the previous 60 years, as long as the next era’s energy resource is not perfected (at which time they would not renew the option for the second 30 years).
My favorite type of semi-passive income was rental property because it was a tangible asset that provided reliable income. As I grew older, my interest in rental property waned because I no longer had the patience and time to deal with maintenance issues and tenants. Online real estate became more attractive, along with tax-free municipal-bond income once rates started to rise.
Rental properties are defined as passive income with a couple of exceptions. If you’re a real estate professional, any rental income you’re making counts as active income. If you’re "self-renting," meaning that you own a space and are renting it out to a corporation or partnership where you conduct business, that does not constitute passive income unless that lease had been signed before 1988, in which case you’ve been grandfathered into having that income being defined as passive. According to the IRS, "it does not matter whether or not the use is under a lease, a service contract, or some other arrangement."
​Network marketing, or multi-level marketing, seems to be on the rise. Companies such as Young Living Oils, Avon, Pampered Chef, and AdvoCare are all multi-level marketing companies. You can earn passive income through network marketing by building a team underneath you (often referred to as a down line.) Once you have a large team you can earn commissions off of their sales without having to do much.
5. Make sure you are properly diversified. Capital preservation is underrated. We saw a lost decade for tech stocks between 2000 and 2010 after the first dot-com bubble burst. It actually took 13 years for Nasdaq investors to get back to even. Investors in the Borsa Istanbul stock market index just gave up 10 years' worth of gains after they saw a plunge in their currency, partially due to increased tariffs by the US and a lack of confidence in the government. Your passive income needs to be properly diversified in order to take the hits.
If you're looking to get started in real estate, look at a crowd funding solution like RealtyMogul. It works similar to LendingClub - you commit as little as $5,000 towards a property. When the property is fully funded, you become an owner, and will receive your share of the earnings and appreciation in the property.  Check out RealtyMogul to learn more.