Editor’s note: Each week, one of our investing experts answers a reader’s question in a Q&A column at our sister site, InvestingAnswers.com. It’s all part of our mission to help consumers build and protect their wealth through education. This week’s question will be answered by Christian Hudspeth…

Question: I’ve read that index funds are the smartest choice for someone like me who’s interested in long-term, low-fee, passive investing. But there are so many to choose from! How do I go about picking a fund that will be worth holding onto for years? — Helen, New Orleans
 
Answer: Great question, Helen. I personally love investing in index funds. They’re an excellent choice for 401(k)s and other retirement plans as well as normal investment accounts — especially if you don’t want to have to spend hours researching your mutual fund performance every quarter.
 
#-ad_banner-#An index fund is simply a mutual fund that tracks a market index and mirrors the performance of that index. So if I invest in the Vanguard 500 Index Fund (Nasdaq: VFINX), which tracks the S&P 500 Index, and the S&P 500 goes up 5%, the Vanguard fund will go up almost 5% as well.
 
I say “almost” because the expense fees, which come along with all mutual funds, will take some of the returns. As most index funds do, the Vanguard 500 Index carries a very low annual expense ratio — 0.17% in this case. So if the S&P 500 went up 5% over the year, my investment would go up 4.83% in this example.
 
Because index funds track market indexes, they don’t have portfolio managers actively buying and selling investments within the funds as regular mutual funds do. And with less activity from the portfolio manager, index funds often carry much lower expense fees and are potentially less risky than normal mutual funds with managers who try timing the market.
 
That also means that you, the investor, don’t need to carefully compare the performance of the fund to the market index’s performance because with an index fund, you’re essentially investing in the market you’re comparing against.
 
In short, index funds are a low-cost, hassle-free way to diversify into several different investment types at once — and for the long-haul.
 
Several brokers, including Vanguard, Charles Schwab, Fidelity and others, offer index funds, all of which are very similar except for one thing — expense fees. You’ll want to look for index funds with the lowest expense ratios, which you can find on every mutual fund’s prospectus page or on Morningstar.com. That can help you keep your costs down and your returns growing as much as possible. Typically, an index fund with an annual expense ratio that’s 0.6% or less is decent, but an expense ratio under 0.25% is ideal.
 
So, how do you choose the “right” index funds?
 
Before we dive into the various types of index funds, let’s talk big picture — your investment mix. Remember, everyone’s investment portfolio is different. Yours should be made up of different investment types that fit your age and risk tolerance, so do your research on portfolio allocation (here’s an article for some guidance) or talk to a trusted advisor for more help on that.
 
For our purposes, I’ll keep this basic.
 
As I talked about in my article, “The Lazy Man’s Retirement Portfolio,” there are basically four major asset classes in which to invest: U.S. stocks, foreign stocks, real estate and bonds. You can also invest in commodity index funds, but they’re a bit outside of the normal asset classes so we’ll skip that one for today.
 
For each of the asset classes you want to invest in, you’ll want an index fund that tracks it.
 
U.S. stocks
If you want exposure to American companies, then you may want an index fund that tracks the Wilshire 5000 index, which includes virtually every small-, medium- and large-cap stock traded in the United States — more than 5,000 companies in all. An example of an index fund that does this is the Vanguard Total Stock Market Index Fund (Nasdaq: VTSMX).
 
If you’re looking to invest only in large, established companies (Wal-Mart, Exxon, Apple, etc.), then you may prefer to go with an index fund that tracks the S&P 500 Index (like the one I mentioned earlier, for example), which tracks the 500 largest companies in the United States.
 
Foreign stocks
For exposure to companies in countries with explosive economic growth, foreign stock index funds are just the ticket. There are plenty of emerging countries index funds to choose from that invest in companies in Mexico, China, Brazil and others, as well as developed countries index funds, which invest in companies in Europe, Japan, Canada and others.
 
If you want exposure to both emerging and developed countries, choose an index fund that tracks the MSCI EAFE index, which follows companies in Europe, Australia and the Far East. An example of an index fund that does this is the Vanguard Total International Stock Index Fund Investor (Nasdaq: VGTSX).
 
Real estate
If you want to own a part of apartment complexes, retirement homes and other kinds of properties, real estate investment trusts (REITS) are the easiest way to do it without having to buy land. To get this kind of exposure, you’ll want an index fund that follows the MSCI US REIT Index, which represents 85% of the REITs in America. An example of a REIT index fund that does this is the Vanguard REIT Index Fund (Nasdaq: VGSIX).
 
Bonds
Finally, if you want some of your investment dollars to be in bonds, you’ll want an index fund that tracks the Barclays Capital Aggregate Bond Index. That index represents most investment-grade bonds traded in the United States: treasury securities, government bonds, mortgage-backed bonds, corporate bonds and even some foreign bonds. An example of a bond index fund that does this is the Vanguard Total Bond Market Index Fund (Nasdaq: VBMFX).
 
Action to Take –> So, three steps in all: Determine the type of asset class in which you’d like to invest, find a brokerage firm that offers low-cost index funds that track those asset classes (the ones mentioned earlier are a good place to start), then just set and forget. Index fund investing is as simple as that.

This article originally appeared on InvestingAnswers.com:
“Ask The Expert: How Do I Pick The Best Index Funds?”