When the news says "the market was up today," they are usually referring to a stock market index. But an index is not the entire market. It is a measurement tool.
What a Stock Market Index Is
A stock market index tracks the performance of a specific group of stocks to represent a section of the market. Think of it like a thermometer for the economy. It does not tell you the temperature of every room. It gives you the average.
Major US Indexes
S&P 500
Tracks 500 of the largest US companies. Market-cap weighted (bigger companies have more influence). Widely considered the best single measure of the US stock market.
Dow Jones Industrial Average (DJIA)
Tracks 30 large US companies. Price-weighted (higher-priced stocks have more influence, regardless of company size). The oldest and most well-known index, but arguably less representative than the S&P 500.
NASDAQ Composite
Tracks all stocks listed on the NASDAQ exchange (over 3,000). Heavily weighted toward technology companies. Often used as a proxy for the tech sector.
Russell 2000
Tracks 2,000 small-cap US companies. Used as a measure of how smaller companies are performing, which can differ significantly from large-cap indexes.
How Indexes Are Weighted
Market-cap weighted (most common): Larger companies have more influence. Used by the S&P 500 and NASDAQ. If Apple moves 2%, it affects the index more than a small company moving 2%.
Price-weighted: Higher-priced stocks have more influence. Used by the Dow Jones. A stock at $300/share affects the index 3x more than a stock at $100/share, regardless of company size.
Equal-weighted: Every stock has the same influence regardless of size or price. Less common but provides a different perspective on market breadth.
Why Indexes Matter for Investors
Benchmarking. Indexes help you measure your portfolio's performance. If your portfolio returned 8% but the S&P 500 returned 12%, you underperformed the market.
Index funds. You can invest in an index by buying an index fund or ETF that replicates its holdings. This is one of the most popular and effective investment strategies.
Market sentiment. Index movements tell you the general direction of the market, though they do not tell you about individual stocks.
What Indexes Do Not Tell You
An index going up does not mean every stock is going up. Market breadth (how many stocks are rising vs falling) can vary significantly even on positive index days. A few large stocks can carry the entire index while the majority decline.
The Progressive Trailblazer helps you research the companies behind any index. Educational only. Not financial advice.