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 growth stocks to your portfolio but don't have the time or expertise to hand-pick a few, the SPDR S&P 400 Mid Cap Growth ETF (NYSEMKT:MDYG) could save you a lot of trouble. Instead of trying to figure out which mid-cap growth 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 growth stocks, sports a low expense ratio -- an annual fee -- of 0.25%. It also recently yielded close to 1%.

This mid-cap growth stocks ETF slightly underperformed the S&P 500 over the past three years but clobbered it over the past five. 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 growth 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. This ETF focuses on mid caps that are growing rather briskly, holding close to 250 mid-cap growth stocks.

Plenty of mid-cap growth stocks had strong performances over the past year. 3-D printing specialist 3D Systems (NYSE:DDD) soared 146%, largely on great expectations. The company is seen by some as overhyped and overvalued (in a richly valued crowd), but others are optimistic about its future, especially due to its recent acquisition spree. For example, it recently snapped up a strategic chunk of Xerox. 3D Systems posted big growth numbers in its third quarter, but also tempered near-term expectations. 3D Systems is significantly shorted, with some worried about Hewlett-Packard's entry into the market. Interested investors should take a long-term view.

LED lighting specialist Cree (NASDAQ:CREE) saw its stock slump following a quarterly earnings report that featured big, double-digit revenue and earnings growth along with reduced near-term projections from management. It's not all bad news, though, as some expect the LED market to grow by about 34% annually over the next few years, eventually totaling nearly $100 billion, and Cree is investing heavily in it. If you've read about the death of the conventional 40-watt and 60-watt light bulb, know that Cree is positioned to profit from that. Cree has introduced a 75-watt LED bulb that, while pricey at $24, is considerably cheaper than those of some competitors. Meanwhile, Cree's bulbs have received an Energy Star rating that will give consumers access to rebates from utility companies. The stock isn't cheap, though, with its P/E ratio topping 70.

Green Mountain Coffee Roasters (UNKNOWN:GMCR.DL), for example, nearly tripled in value (and is still actually about 25% below its 52-week high). Part of its success is tied to its Keurig machines and some auspicious licensing agreements. (An intriguing recent deal is with Campbell Soup to develop K-Cup soups.) Green Mountain Coffee Roasters faces competition from private labels, and now Whole Foods Market is getting into the act, too. Green Mountain Coffee Roasters stock yields 1.3%. The company's future is impossible to see clearly, but there are many reasons to be hopeful, and the stock seems appealingly priced, with its forward P/E ratio in the teens.

Polaris Industries (NYSE:PII), which specializes in snowmobiles and on- and off-road vehicles, gained 72%. Bulls like its smart management, growing market share, and savvy recent acquisitions. With its Indian and Victory motorcycles, it's seen as capable of outperforming Harley-Davidson. Polaris Industries' stock yields 1.2%, and that payout has more than doubled in about three years. Its forward P/E, near 18, seems reasonable, given its robust growth rates. (Polaris Industries' third quarter featured revenue and EPS from continuing operations up 25% and 23% over year-ago levels, respectively.)

Other mid-cap growth stocks didn't do quite as well over the last year, but could see their fortunes change in the coming years. Trimble Navigation, for example, gained about 13%, but that's well below the S&P 500, which advanced close to 30%.

The big picture
If you're interested in adding some mid-cap growth 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 it and profiting from it that much easier.