Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you're looking for some sturdy but still growing giants to add to your portfolio, the Vanguard Mega Cap 300 Growth Index ETF (NYSE: MGK) could save you a lot of trouble. Instead of trying to figure out which companies 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. The Vanguard ETF's expense ratio -- its annual fee -- is a very low 0.12%.

This ETF has performed reasonably well, beating the S&P 500 over the past three years and also so far this year. 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.

With a low turnover rate of 26%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Plenty of huge and growing companies had strong performances over the past year. Philip Morris International (NYSE: PM), for example, surged 44%. Operating outside the U.S., it avoids our increasing regulations and shrinking smoking base, but some worry that a bigger meltdown in Europe could hurt the company. Bulls like how the company is expanding its Marlboro brand with new flavors, package sizes, and more in order to grab more consumers.

Boeing (NYSE: BA), up 22%, has taken to the skies after finally bringing its Dreamliner 787 to market. The company recently announced 6% price hikes on all other planes, though that won't have much of an immediate impact since it deals with long-term contracts and backlogs. It's also racked up new orders recently, for nearly 100 planes.

Data storage titan EMC (NYSE: EMC) gained 17%, recently racking up revenue gains of 10% and expanding its operations in flash storage, among other things. It has also partnered with Lenovo to provide storage in China, and its management recently noted, "We believe we have the right strategy in place to leverage the 3 major waves of change in IT: cloud, Big Data and trust." Bulls also like its majority ownership of cloud computing specialist VMware.

Occidental Petroleum (NYSE: OXY), meanwhile, advanced 10%, despite recently reporting sagging revenue and earnings, due to declining oil prices. The company continues to invest in new exploration and development, though, and is poised to perform well when conditions improve. Its profit margins have generally been rising over the past few years, with net margins recently near a hefty 26%.

The big picture
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.

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