Introduction
Index ETFs are designed to follow the performance of a market index. Instead of a fund manager choosing individual investments based on forecasts or opinions, the ETF generally holds securities according to rules established by the index it tracks.
Some index ETFs provide broad exposure to hundreds or thousands of companies. Others track narrower indexes focused on a particular country, company size, sector, bond market, or investment style.
Because of their simplicity and often relatively low costs, index ETFs are widely used as building blocks in long-term portfolios.
What is an index ETF?
An index ETF is an exchange-traded fund that seeks to replicate, as closely as practical, the performance of a specified index before fees and expenses.
An index itself is a rules-based collection of securities used to measure a particular market or segment. Well-known examples include the S&P 500, which represents large U.S. companies, and indexes covering broader U.S. or international markets.
Investors cannot invest directly in an index. An index ETF provides a practical way to gain exposure to the securities represented by that index.
How index ETFs track an index
One method is full replication. The ETF buys the securities in the index in approximately the same proportions as the index itself.
For indexes containing many securities, a fund may instead use sampling. The ETF holds a representative selection designed to produce performance similar to the full index without owning every security.
Some specialized funds may also use derivatives as part of their tracking approach. The fund’s prospectus explains how it seeks to achieve its objective.
Common types of index ETFs
Broad-market ETFs
These funds track indexes covering a large portion of a stock or bond market and can provide extensive diversification through one investment.
Large-cap ETFs
These ETFs follow indexes of larger companies, such as indexes representing major U.S. corporations.
International ETFs
International index ETFs can provide exposure to developed markets, emerging markets, individual countries, or combinations of global markets.
Bond index ETFs
These funds track indexes composed of government, corporate, municipal, or other fixed-income securities.
Advantages of index ETFs
Broad index ETFs can make diversification relatively simple. Instead of researching and purchasing many individual companies, an investor can gain exposure to an entire group of securities through one fund.
Index strategies also tend to require less security selection and trading than many active strategies. This can contribute to lower operating costs, although expense ratios vary between funds.
Another advantage is transparency of strategy. Because an index follows published rules, investors can usually understand the type of securities a fund is designed to own and why they are included.
Risks and limitations
An index ETF follows its index in both rising and falling markets. It does not move to cash simply because the fund manager expects prices to decline.
Diversification also depends on the index. A total-market ETF can be broadly diversified, while an index covering one industry may remain highly concentrated.
Indexes can also be weighted differently. Market-cap-weighted indexes give larger companies greater influence, while equal-weighted and factor-based indexes use different rules. These choices can materially affect a fund’s performance and risk.
Tracking error
An index ETF rarely produces exactly the same return as its benchmark. The difference between the fund’s performance and the index can be influenced by expenses, transaction costs, sampling, taxes, cash holdings, and portfolio management.
This difference is often discussed as tracking difference or tracking error. When comparing similar index ETFs, investors may look at both fees and how consistently each fund has tracked its benchmark.
What to compare between index ETFs
| Factor | Why it matters |
|---|---|
| Underlying index | Determines which securities the ETF is designed to hold |
| Expense ratio | Reduces the return investors receive from the portfolio |
| Diversification | Shows how broadly exposure is spread across securities and markets |
| Weighting method | Determines how much influence each holding has |
| Tracking quality | Shows how closely the ETF has followed its benchmark |
| Liquidity and spread | Can affect the cost of buying and selling shares |
Are index ETFs passive investments?
Index ETFs are generally described as passive because the portfolio follows an index rather than relying on a manager to select securities actively. However, the index itself still reflects a set of choices, including which market to cover, how securities qualify, and how they are weighted.
For that reason, investors should understand the index rather than assuming all index ETFs provide the same type of passive exposure.
Key takeaways
- Index ETFs seek to track the performance of a specified market index.
- They can provide broad market exposure or focus on narrower segments such as sectors, countries, or company sizes.
- Funds may use full replication, sampling, or other techniques to follow an index.
- An index ETF can still lose value when its underlying market declines.
- The index’s weighting method and concentration can significantly affect the fund’s risk and performance.
- Expense ratios, tracking quality, liquidity, and the underlying index are important factors when comparing funds.