Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add an index fund to your portfolio that focuses on large-cap companies and weights them  by earnings, not market capitalization, the WisdomTree Total Earnings ETF (NYSEMKT:EXT) might serve you well. Instead of trying to figure out which companies have high earnings, 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 WisdomTree ETF's expense ratio -- its annual fee -- is a low 0.28 %. It even yields about 1.9%. The fund is fairly small, though, 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's performance  is generally positive, beating the S&P 500 over the past five years, but slightly underperforming 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.

With a low turnover rate of 12 %, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

Why high earnings?
It can be good to focus on earnings over market capitalization, because market caps can reflect more enthusiasm or pessimism than actual performance. More than a handful of high-earnings companies  had strong performances over the past year. Pfizer (NYSE:PFE) soared 40%, but its future is challenged by the patent expiration on some key drugs, such as Lipitor. The company has boosted earnings lately, but largely because of cost-cutting in the face of falling Lipitor sales. All is not lost, though, as approvals for drugs in its pipeline can make a big difference, and news is due soon  for its rheumatoid arthritis drug Tofacitinib.

Coca-Cola (NYSE:KO) advanced 12%, recently reporting slow growth, partly because of the strong dollar. Some investors are excited about its plans to buy back more than 11% of its stock, but others note that the stock isn't near bargain levels right now, so buying richly valued shares isn't the smartest move. Meanwhile, the beverage company is looking into buying music-streaming company Spotify, leaving some scratching their heads.

Other companies didn't do as well last year but could see their fortunes change in the coming years. Procter & Gamble (NYSE:PG) gained just 7% over the past year, with its revenue growing very slowly and its net income shrinking in recent years. The company reports its latest quarterly numbers on or about Oct. 25, and many investors would like to see the company trim some fat and start growing its top line again in the near future. While they wait, though, they can collect a 3.3% dividend yield.

Oracle (NYSE:ORCL) dropped 3%, but has been successfully shifting its focus from hardware to the cloud computing realm. Some dismiss the company prematurely, thinking of CEO Larry Ellison's lavish ways, such as his purchase of an entire Hawaiian island. But others, such as Wall Street analysts who have raised their projections for the company, have high hopes.

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 Coca-Cola and Procter & Gamble. The Motley Fool owns shares of Oracle. Motley Fool newsletter services recommend Coca-Cola and 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.