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 stocks to your portfolio but don't have the time or expertise to hand-pick a few, the Vanguard Mid-Cap ETF (NYSEMKT:VO) 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 stocks, sports a tiny expense ratio -- an annual fee -- of 0.1%.

This mid-cap stocks ETF has outperformed the S&P 500 over the past five and 10 years. 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 stocks?
Mid-cap stocks can be wonderful additions to portfolios because they've proven themselves to some degree, having grown to mid-cap size. And better still, they also still have a lot of room to grow -- before they become large caps.

More than a handful of mid-cap stocks had strong performances over the past year. Stent and defibrillator specialist Boston Scientific Corporation (NYSE:BSX), for example, surged 76%. The company topped earnings expectations in its fourth quarter, but stent sales were disappointing. Indeed, profits have been a bit elusive for the company in the past few years. Bulls like Boston Scientific's growth trends and its under-control debt levels, and they see the company successfully turning itself around. Additionally, Boston Scientific's newer technologies, such as neurotech devices, have been paying off. Endoscopy sales recently rose 8%. Analysts at Zacks Equity Research like its "strong pipeline of products under development."

Seagate Technology PLC (NASDAQ:STX), with its enterprise server drives, business data storage, and other data storage offerings, popped 47% and yields 3.4%. (It has been upping its payout by an annual average of 29% over the past five years, most recently by 13%.) With rival Western Digital, it controls 85% of the hard-drive market, and with its purchase last year of Xyratex, a hard-drive testing specialist, it turned Western Digital into a customer. A shift toward 3D NAND in the flash memory industry is likely to boost Seagate's fortunes. Still, it's hurt by a weak PC market and its profit margins have taken a hit, too. Its fourth quarter was disappointing.

Flash memory specialist SanDisk Corporation (NASDAQ: SNDK) gained 34% and also stands to gain from a slow shift toward 3D NAND memory. Its solid-state drive (SSD) business is growing briskly, and has benefited from the spread of mobile technology. Bulls like its prospects in China -- SanDisk already gets more than half its revenue from emerging markets. Its fourth quarter was strong, featuring revenue rising 12% over year-ago levels and non-GAAP earnings soaring more than 50% as profit margins grew. Bears worry about pricing pressures and whether the company can keep costs under control. SanDisk initiated a dividend last year, and it recently yielded 1.2%.

Other mid-cap stocks didn't do quite as well over the last year, but could see their fortunes change in the coming years. Health Care REIT (NYSE:WELL), for example, shed 6%. Its prospects are bright, as the oldest Baby Boomers are entering or approaching retirement years and Health Care REIT focuses on buying senior-living facilities as well as hospitals and other medical properties, which it then leases back to occupants. It offers a fat 5.4% dividend yield, though it has been paying out more than it brings in via earnings lately, which is not sustainable over the long run. Still, the company has been growing aggressively via acquisitions. In its fourth quarter, funds from operations grew by 16% over year-ago levels, and full-year FFO rose 8%. The quarter also featured nearly $278 million spent on 12 property acquisitions and total revenue surging some 59%.

The big picture
If you're interested in adding some mid-cap 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.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.