Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some mid-cap value stocks to your portfolio but don't have the time or expertise to hand-pick a few, the SPDR S&P 400 Mid Cap Value ETF (NYSEMKT:MDYV) could save you a lot of trouble. Instead of trying to figure out which stocks will perform best, you can use this ETF to invest in lots of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. This ETF, focused on mid-cap value stocks, sports a relatively low expense ratio -- an annual fee -- of 0.28%. The fund is small, so if you're thinking of buying, beware of possibly large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

This mid-cap value stocks ETF has outperformed the S&P 500 over the past five years but lagged it over the past three. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

Why mid-cap value stocks?
Mid-cap value stocks can be wonderful additions to portfolios because they've proven themselves to some degree, having grown to mid-cap size. They also still have a lot of room to grow before they become large caps. And better still, the mid-cap value stocks in this ETF are there because they exhibit some characteristics suggesting that they're undervalued.

More than a handful of mid-cap value stocks had strong performances over the past year. New York Community Bancorp, Inc (NYSE:NYCB) popped 23% and yields a tasty 6.2%. It has grown faster and more profitably than many peers through acquisitions of other banks and growth in its commercial and industrial lending business. It has a strong long-term track record, in part due to a focus on financing rent-controlled buildings in New York City and successfully avoiding many bad loans. The bank's fourth quarter featured earnings down a bit from year-ago levels, in part due to a fall in mortgage volume.

Raymond James Financial (NYSE:RJF) jumped 19% and yields 1.2%. Its first quarter was strong, topping both earnings and revenue expectations. Its 6% revenue growth was due in large part to fee increases. Management noted, "This marks our 104th consecutive quarter of profitability, a record we're proud of, especially in our industry." CEO Paul Reilly was also refreshingly candid, saying, "But there was also a number of favorable items that all just happened to hit this quarter that made a strong quarter look, from an ongoing operating basis, maybe a little stronger than it really was." These included some one-time fees. Raymond James Financial's challenges include low interest rates and an economy that isn't yet firing on all cylinders.

Other mid-cap value stocks didn't do quite so well over the last year but could see their fortunes change in years to come. HollyFrontier Corporation (NYSE:HFC) gained 4% and yields 2.5%. HollyFrontier has reduced refining capacity at its Navajo refinery due to waste water constraints, but that's a short-term problem. Bulls like the company's prospects from the Barnett Shale region, as well as its cash generation, solid balance sheet, and special dividends. Its fourth quarter featured revenue and earnings dips, but many still see the company as well managed.

Realty Income Corp (NYSE:O) shed 6% and yields 5.4%, with its dividend paid monthly and not quarterly. Its fourth quarter featured many solid numbers, such as high occupancy rates and improving diversification. Bulls like its easy-to-understand business model, among other things as it actually owns properties and collects rent payments on them.

The big picture
If you're interested in adding some mid-cap value stocks to your portfolio, consider doing so via an ETF. A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.