Stock Market Index: What It Is and How It Works
- What Is a Stock Market Index?
- Why Are Stock Market Indexes Used?
- How Is a Stock Market Index Constructed?
- Market-Capitalization-Weighted Indexes
- Price-Weighted Indexes
- Equal-Weighted Indexes
- Indexes and Investment Funds
- Does an Index Show the Entire Market?
- Indexes Can Change Over Time
- Stock Market Indexes and Investment Risk
- Key Takeaways
A stock market index is a group of selected stocks used to represent the performance of a particular market, sector or group of companies. Indexes are commonly used to understand broad market movements and to compare investment performance.
What Is a Stock Market Index?
A stock market index tracks the combined performance of a selected group of stocks according to a defined methodology. The companies included in an index are chosen based on rules set by the organization responsible for maintaining that index.
An index does not represent every company in the stock market. Instead, it provides a way to measure the performance of a specific group of securities.
A stock market index is like a measurement tool that helps show how a selected group of stocks is performing over time.
Why Are Stock Market Indexes Used?
Indexes can help investors, analysts and market participants understand general market trends. Instead of examining hundreds or thousands of individual stocks, an index can provide a broad view of how a selected part of the market is moving.
Indexes may also be used as benchmarks when comparing the performance of investment portfolios or funds.
How Is a Stock Market Index Constructed?
Different indexes use different rules. The methodology may define which companies are eligible, how often the index is reviewed and how much influence each company has on the index value.
Some indexes use market capitalization as part of their methodology, while others may use different weighting approaches.
Market-Capitalization-Weighted Indexes
In a market-capitalization-weighted index, companies with larger market capitalizations generally have a greater influence on the movement of the index than smaller companies.
This means that a significant price movement in a large company may have a larger effect on the index than a similar movement in a smaller company.
Price-Weighted Indexes
Some indexes give greater influence to stocks with higher share prices. In these indexes, the share price can play an important role in determining how much a company's price movement affects the overall index.
Equal-Weighted Indexes
An equal-weighted index gives each included company approximately the same weight at the time of rebalancing. This means that smaller companies may have a greater relative influence compared with a market-capitalization-weighted approach.
Indexes and Investment Funds
Some investment products are designed to follow or track the performance of a particular index. These may include certain index funds and exchange-traded funds.
However, an investment fund may not exactly match the performance of its benchmark because of costs, fees, trading expenses, portfolio differences and other factors.
If an index rises over a certain period, it indicates that the combined value of the stocks represented by that index has generally increased according to its methodology. Individual stocks within the index, however, may have performed very differently.
Does an Index Show the Entire Market?
Not necessarily. An index only represents the securities included under its methodology. A broad market index may represent a large part of a market, while a sector-specific index may focus only on companies from a particular industry.
It is important to understand what an index includes before using it to draw conclusions about the overall market.
Indexes Can Change Over Time
The companies included in an index may change. Index providers can periodically review eligibility and make additions or removals according to their published methodology.
Changes in company size, market conditions, corporate actions or other factors may affect index composition.
Stock Market Indexes and Investment Risk
An index moving higher or lower does not guarantee future market performance. Markets can change because of economic conditions, company earnings, interest rates, investor expectations and many other factors.
Investing in a fund that tracks an index can also involve risk, including the possibility of losing money.
Key Takeaways
- A stock market index measures the performance of a selected group of stocks.
- Different indexes use different rules and methodologies.
- Indexes can be weighted by market capitalization, share price or other approaches.
- Indexes are commonly used to understand market movements and compare investment performance.
- An index does not guarantee future returns or eliminate investment risk.