Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect seemingly undervalued large-cap companies to prosper as their intrinsic value is eventually realized and as they grow even bigger over time, then the First Trust Large Cap Value AlphaDEX ETF (NYSE: FTA) 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 First Trust ETF's expense ratio -- its annual fee -- is 0.70%, which is a bit higher than many ETFs, but still lower than the typical stock mutual fund.

This ETF has performed reasonably well, beating the S&P 500, on average, over the past three years. 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.

What's in it?
Several of this ETF's components made strong contributions to its performance over the past year. Intel (Nasdaq: INTC), up 15% this year, is maintaining its chip dominance and benefiting from growing PC demand in emerging markets and strong demand for servers, as well.

Other companies didn't add as much to the ETF's returns last year, but could have an effect in the years to come. Xerox (NYSE: XRX), down about 31%, isn't your father's copier company anymore. It's now developing new technologies such as image-recognition software -- which complement reliable office-machine service contract revenue. Weyerhaeuser (NYSE: WY), down 10%, has some investors excited about its rich timberland holdings and how they're a good countercyclical hedge against inflation.

Glass and fiber giant Corning (NYSE: GLW) lost about 31%, despite seeing demand for its Gorilla Glass grow for inclusion in the displays of tablets, smartphones, and other devices. Semiconductor equipment maker Applied Materials (Nasdaq: AMAT), down 24%, has been cited as one of several "stocks that have it all" by my colleague Dan Caplinger -- sporting a reasonable valuation, a healthy expected growth rate, and an attractive dividend. Indeed, it's been hiking its payout by an annual average of 10% over the past five years.

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.

Learn about the best dividend ETFs. And if you're looking for some great investments beyond ETFs, consider these 10 Stocks for Your Retirement Portfolio.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.