In its analysis, the Irvine, Calif.-based firm looked at the returns an investor would make on the purchase and rental of a three-bedroom family home or condo in the 285 counties that had at least 100,000 residents and enough data about home purchase and rental prices in order to discern trends.

Blomquist explained that high rents are not necessarily the key factor that gives an area a strong profile for rental returns, as many people assume. Rather, it's the cost of purchasing real estate an investor would offer for rent.

"We don't really do anything too complicated in this report," Blomquist said, referring to the report's rent versus buy analysis. "We really look at the simplest measurement of rental return – the annual gross rental yield."

Annual gross rental yield is the amount an investor would receive from a year's rent from a given property, divided by what it cost to purchase that property and prepare it for tenants. So, for a property that an investor purchased for $100,000 and rented out for $1,000 per month, the annual gross rental yield would be 12% ($12,000 divided by $100,000).

This calculation explained why certain counties are high on the list of good rental counties for investors, Blomquist said. Their rents might not be terribly high, but their costs to buy into the rental market were low enough for them to generate strong rental yield.

Some of those counties included Clayton County, Ga. in the Atlanta metropolitan area and Bay County, Mich. in the Bay City, Mich. metropolitan area.

Although rents in those areas might seem very inexpensive compared to those in a hot housing market such as San Francisco, New York City or Washington, they were still strong enough to generate good rental yield, Blomquist explained. According to the report, Clayton County had an average annual gross rental yield of more than 24%. Bay County's was more than 19%, according to the report.

Many people got into rental real estate in the years immediately following the housing downturn, in part because the very low costs of real estate purchases made their annual gross rental yields extraordinarily high, Blomquist explained.

According to the firm's figures, average annual gross rental yields surpassed 10% in 2011 and have been falling slowly but steadily ever since, to the point where investors might begin to consider whether keeping a piece of real estate as a rental represents the best use of that property.

Blomquist pointed out that annual gross rental yield does not include ongoing costs of maintaining or managing a property or its rentals.

The researchers also looked at areas where, from a consumer perspective, the costs of renting surpassed the costs of buying.

Residents of roughly two-thirds of counties with populations of more than 100,000 would find it less expensive to buy a home than to rent one, according to the research. Counties where it has become more affordable to buy a home than to rent one include Miami-Dade County, Fla.; San Bernardino County, Calif.; Clark County, Nev.; Broward County, Fla. and Wayne County, Mich.

RealtyTrac also identified 13 major counties where, it said, it had been more affordable to rent than buy in 2014, but had since become more affordable to buy than rent. Those include counties located near Seattle, Wash.; Reading, Penn.; Indianapolis, Ind.; Olympia, Wash. and Cincinnati, Ohio.

Conversely, the firm found it is more affordable to rent than buy in the remaining one-third of counties surveyed. Many of these counties are in California and include Los Angeles, San Diego, Orange and Riverside.

King County, Wash. in the Seattle area and Denver County, Colo. were two non-Californian counties where it was still less expensive to rent than buy a home, according to the firm.