What Is A Stock Market Index?
A stock market index is a way to measure the overall performance of a group of selected stocks. Think of it like a scoreboard for a particular part of the market—it shows whether that group of companies is gaining or losing value over time.
Indexes help investors:
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Track the market’s performance
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Compare how their investments are doing
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Understand how the economy or certain industries are behaving
How Indexes Work
Each index includes a set number of stocks, chosen to represent a certain sector, region, or the entire market. The index value rises or falls based on the performance of those stocks.
For example:
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If most companies in the index increase in value, the index goes up.
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If they lose value, the index goes down.
Indexes can be price-weighted, market-cap weighted, or equal-weighted—which just refers to how each company’s influence on the index is calculated.
Major Stock Market Indexes Around the World
Here are some of the most widely followed indexes and what they represent:
S&P 500 (USA)
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Tracks the 500 largest publicly traded companies in the U.S.
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Covers around 80% of the total U.S. stock market.
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Considered the best single indicator of the U.S. stock market’s health.
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Includes companies like Apple, Microsoft, Amazon, Google (Alphabet), and Tesla.
Dow Jones Industrial Average (DJIA)
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Includes 30 large, blue-chip companies in the U.S.
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One of the oldest indexes (founded in 1896).
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Price-weighted—so higher-priced stocks have more impact on the index.
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Companies include Coca-Cola, Boeing, McDonald’s, and Goldman Sachs.
NASDAQ Composite
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Contains over 3,000 companies, mainly in the tech sector.
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Includes giants like Apple, Meta (Facebook), Netflix, Nvidia, and Amazon.
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Known for being tech-heavy and more volatile.
ASX 200 (Australia)
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Tracks the top 200 companies listed on the Australian Securities Exchange.
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Includes major Australian companies like BHP, Commonwealth Bank, CSL, and Woolworths.
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Seen as the main benchmark for the Australian economy.
FTSE 100 (UK)
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Represents the 100 largest companies on the London Stock Exchange.
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Includes international names like Shell, HSBC, GlaxoSmithKline, and British American Tobacco.
Nikkei 225 (Japan)
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Tracks 225 leading companies on the Tokyo Stock Exchange.
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Includes major Japanese brands like Toyota, Sony, Nintendo, and Panasonic.
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Similar in structure to the U.S. Dow Jones index.
Shanghai Composite Index (China)
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Includes all stocks (A-shares and B-shares) listed on the Shanghai Stock Exchange.
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Heavy with large Chinese firms like PetroChina, ICBC, and China Life Insurance.
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Reflects the state of China’s more state-influenced economy.
EURO STOXX 50 (Europe)
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Tracks the top 50 companies in the Eurozone (countries that use the Euro).
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Includes major firms like LVMH, Siemens, Allianz, and Volkswagen.
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Useful for understanding broader European market trends.
Why Indexes Matter
Indexes are used for:
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Benchmarking – comparing how individual investments perform.
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Investing – many investors buy index funds or ETFs that track these indexes, offering instant diversification.
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Economic Signals – indexes often reflect the health of an economy or sector.
When news reports say “the market was up today,” they usually mean that one or more major indexes rose.