Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some large companies to your portfolio, the SPDR Dow Jones Large Cap ETF (NYSEMKT:ONEK) 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 SPDR ETF's expense ratio -- its annual fee -- is a very low 0.2%. The fund is fairly small, too, 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 ETF has roughly matched  the S&P 500 over the past three and five years, and recently yielded about 1.9%. 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 4%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

Why large companies?
It's best to keep your portfolio diversified, by industry, geography, and market capitalization. Large companies can add some ballast to your collection. Many may not grow as briskly as their smaller counterparts, but to reach their current size, they probably have some strong assets and features.

More than a handful of large-cap companies  had strong performances over the past year. General Electric (NYSE:GE) surged 36%, as it expands into alternative energies and mining, while seeing certain divisions, such as GE Capital, get healthier. Its massive cash hoard gives it the ability to seize opportunities (such as the LED lighting fixtures company it recently bought), but the stock isn't quite the bargain it was last year.

Philip Morris International (NYSE:PM) gained 24% and sports generally brighter prospects than its domestic counterpart, Altria (NYSE:MO), which faces increasingly restrictive legislation and rising taxes in many locations. Philip Morris faces those issues, too, but to a lesser degree than in the U.S., where the base of smokers is shrinking as well. In its last quarter, Philip Morris reported a 6% drop  in income because of shrinking volume despite higher prices. Patient investors can collect a 3.8% dividend yield here.

Procter & Gamble (NYSE:PG) advanced 12%. It recently yielded an attractive 3.2%, but some don't see it as much of a bargain, despite some nods from Wall Street analysts. For one thing, many of its products are rather mature, as is the U.S. market. P&G needs to innovate successfully and make the most of growth abroad, particularly in developing economies. The company is no stranger to innovation, though, and it's likely to keep growing for quite a while.

Other companies didn't do as well last year but could see their fortunes change in the coming years. Oracle (NYSE:ORCL) gained just 4%, successfully shifting its focus from hardware to the cloud computing realm. Some Wall Street analysts have raised their projections for the company, while others remain cautious. With a recent P/E of 16 and a forward P/E of 11, this growing software company seems rather attractive -- and would be more so if it upped its anemic dividend.

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.

Longtime Fool contributor Selena Maranjian, whom you can follow on Twitter, owns shares of Procter & Gamble. The Motley Fool owns shares of General Electric and Oracle. Motley Fool newsletter services recommend Procter & Gamble. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.