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    Thomas J. Brock, CFA®, CPA

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    Thomas Brock, CFA®, CPA, is a financial professional with over 20 years of experience in investments, corporate finance and accounting. He currently oversees the investment operation for a $4 billion super-regional insurance carrier.

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  • Updated: December 19, 2022
  • 4 min read time
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How to Cite's Article

APA Brock, T. J. (2022, December 19). What Is an Index? Retrieved January 31, 2023, from

MLA Brock, Thomas J. "What Is an Index?", 19 Dec 2022,

Chicago Brock, Thomas J. "What Is an Index?" Last modified December 19, 2022.

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Index Definition

An index, by its definition, provides an indicator or measure of something. In the investment world, it’s a statistical tool that tracks the performance of an asset class or a sector of an asset class. Today, there are indexes for nearly all aspects of the capital markets.

At a high level, indexes typically fit into one of a handful of broad personal finance categories, including stocks, bonds, real estate and commodities, but these categories can be segmented into much narrower niches. A handful of the better known indexes, all of which track publicly-traded securities, are outlined below.

Widely Known Securities Indexes
S&P 500 Index
The S&P 500 tracks performance of the common stock issuances of the 500 largest companies in the U.S., which are often referred to as “blue chips stocks.”
Nasdaq Composite Index
The Nasdaq composite index tracks performance of the common stock issuances of over 3,000 technology-related companies in the U.S.
This index tracks the stock issuances of large- and mid-cap stocks across 23 developed markets and 25 emerging markets. It seeks to represent the universe of global, publicly traded stocks.
Bloomberg Barclays U.S. Aggregate Bond Index
Often referred to as the Barclays Agg, this index tracks the investment-grade, U.S. dollar-denominated, fixed-rate taxable bond market. The index includes Treasuries, government-related and corporate securities, residential mortgage-backed securities, asset-backed securities and commercial mortgage-backed securities.
S&P GSCI Commodity Index
The S&P GSCI commodity index is the first broad-based and investable commodity index that offers a reliable representation of the universe of global commodities.

How Are Indexes Used?

An index is a statistical tool that helps economists, investors and businesspeople speak the same financial language. But how is the tool actually used by investors? Essentially, there are three main applications of indexes.

Market Monitoring

The most common usage of an index is to monitor market performance and gauge economic sentiment. For instance, the S&P 500 index, which represents approximately 80 percent of U.S. publicly traded stocks, is a very sound measure of domestic stock market performance and economic sentiment. Individuals and organizations all over the world monitor this index to get a quick indication of how things are going in the U.S.


Performance Benchmarking

Beyond monitoring the market at a high level, many investors use indexes to gauge the performance of specific investments. For example, an active investor in the U.S. stock market can use the S&P 500 index to evaluate whether he or she is performing favorably or unfavorably to the broader market. Favorable relative performance is indicative of shrewd decision-making, while underperformance is indicative of suboptimal trading and poor market timing.

Performance benchmarking is especially important when evaluating the performance of actively managed mutual funds and other investment vehicles. Here, benchmarking gives investors the ability to determine whether investment managers, who can be very expensive, are performing well.

Index-Linked Investing

Arguably the most beneficial application of an index is index-linked investing — commonly referred to as passive investing. It is a widely popular and low-cost way to replicate the returns of an index rather than trying to outperform it.

Index-linked investing happens when an investment fund or strategy is established with positions that mirror an index. For example, the Dow Jones Industrial Average (DJIA) is replicated by buying proportionate positions in the 30 companies in the DJIA and maintaining the appropriate proportions over time. Essentially, this means buying more of the stock of outperforming companies and selling more of the stock of underperforming companies.

Pro Tip
Investing in an index fund or exchange traded fund (ETF) can help you avoid all of the research and administrative work necessary to maintain index-matching positions.

Investment companies recognized the merits of index-linked investing several decades ago. Since then, a myriad of index funds and ETFs have emerged, providing access to virtually every market and market sector in the world. Nevertheless, the number of indexes is continually increasing, largely because there is an insatiable appetite for new ways to invest in capital markets.

What Is an Indexed Annuity?

Index-linked investing is not only associated with index funds and ETFs. It is a prominent feature of indexed annuities, which are financial contracts issued by insurance companies to individuals looking to save for retirement. The typical indexed annuity works as follows:

In exchange for an upfront payment, the insurer provides the individual — or annuitant — a guaranteed stream of future income, with the opportunity to increase future payouts based on the performance of a stated index such as the MSCI ACWI. When the index performs well, the future income stream grows (up to stated limits), but the value of the annuity never declines.

As such, indexed annuities provide retirement savers the chance to share in market gains while limiting their downside risk.


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Last Modified: December 19, 2022

7 Cited Research Articles writers adhere to strict sourcing guidelines and use only credible sources of information, including authoritative financial publications, academic organizations, peer-reviewed journals, highly regarded nonprofit organizations, government reports, court records and interviews with qualified experts. You can read more about our commitment to accuracy, fairness and transparency in our editorial guidelines.

  1. Bloomberg. (n.d.). Bloomberg US Agg Total Return Value Unhedged USD. Retrieved from
  2. Corporate Finance Institute. (2020, February 25). Active Management. Retrieved from
  3. MSCI. (n.d.). MSCI ACWI Index. Retrieved from
  4. Nasdaq. (n.d.). NASDAQ Composite Index. Retrieved from
  5. S&P Dow Jones Indices. (n.d.). S&P 500. Retrieved from
  6. S&P Dow Jones Indices. (n.d.). S&P GSCI. Retried from
  7. U.S. Securities and Exchange Commission. (n.d.). Index Funds. Retrieved from