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Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some health-care stocks to your portfolio, the iShares Dow Jones U.S. Health Care ETF 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.

This ETF has performed reasonably, beating the world market over the past three and five years, but slightly underperforming it over the past 10. 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 7% , this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

Why health care? For starters, our planet's growing and aging population will keep demand rising for health-care products and services. On top of that, Obamacare is likely to usher more Americans into health coverage, delivering more consumers to companies treating them.

More than a handful of health-care companies had strong performances over the past year. Celgene gained 35%. It sports five-year average annual revenue growth rates of more than 30%, and its three-year average earnings growth is 36%. The only bad news in a sea of good recently was the slight lowering of sales projections for its multiple myeloma drug, Revlimid -- and approval for it in China is expected soon, too. Bulls like the good results so far for its pancreatic drug, Abraxane. It also received a Wall Street upgrade recently.

Abbott Labs advanced 26%, and has just split its pharmaceutical business from its nutrition and devices businesses, which some expect will unlock more value for investors. The new pharmaceutical entity is AbbVie , starting out with about $18 billion in annual revenue, but also a lot of debt. Some worry that AbbVie is too dependent on its $8 billion drug Humira. It does have other drugs in its pipeline, but they're not approved and selling yet, and some may never get that far.

Abbott Labs, meanwhile, has its own appeal, offering diversification as it focuses on nutrition, diagnostics, devices, and generic pharmaceuticals. It also sports significant growth potential, from emerging markets -- which are expected to contribute 50% of sales by 2015. Abbott recently yielded 1.7%, while AbbVie yielded 4.6%.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Intuitive Surgical , a dominant force in robotic surgical machines, gained 7%, as more hospitals bought its equipment and then proceeded to generate recurring revenue for the company by buying service contracts and consumable accessories and supplies for the machines. Intuitive recently got clearance to offer surgical stapling products, which is a lucrative new arena for it. Bears worry, though, about pressure to reduce health-care costs, and the company has received some negative press and a negative forecast from Citron Research.

The big picture Demand for health-care products and services isn't going away anytime soon. 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.