Understanding Index Trading: What Are Stock Indices and Their Uses?

This article explains what a stock index is in the context of index trading, detailing its purposes as both an investment instrument and a benchmark for financial markets and the economy. It delves into how a stock index tracks the prices of various stocks, highlighting its significance in the world of finance

index trading

What is a stock index?

A stock index is a tool that measures the performance of a segment of the stock market. It is computed from the prices of selected stocks, typically a weighted average. It's used by investors and financial managers to describe the market and compare the return on specific investments.

An index is created by selecting a group of stocks that are representative of a particular market or a sector of the economy. For instance, the S&P 500 is an index of 500 large companies listed on stock exchanges in the United States, representing a broad spectrum of the U.S. economy. Similarly, the Dow Jones Industrial Average (DJIA) includes 30 prominent companies from the U.S. stock market.

Stock indexes are useful for investors as benchmarks to gauge the performance of their portfolios. They also help in understanding market trends and making comparisons across different segments of the stock market. Indexes can also form the basis for investment products like index funds and exchange-traded funds (ETFs).

Stock indices have, among others, the following features:

  • They offer transparency by representing the collective performance of a group of stocks.
  • Indices reflect market perceptions of the companies and economic performance.
  • Indices are the foundation for creating investment products like ETFs.
  • They serve as tools for predicting market trends and analyses.
  • Indices aid in asset allocation within investment portfolios.
  • They serve as a basis for economic and political decisions.
  • Indices are used for efficient risk management in portfolios.
  • They reinforce economic research by providing valuable data.

Structure of a stock index

The structure of a stock index resembles that of a basket, incorporating various products and aggregating their prices to represent an overall market situation.

The structure of a UK stock index, such as the FTSE 100, is designed to reflect the performance of a specific segment of the stock market in the United Kingdom. Here are the key components of its structure:

  • The index is composed of stocks from a select number of companies. For instance, the FTSE 100 includes the 100 largest companies by market capitalisation listed on the London Stock Exchange (LSE).
  • The stocks in the index are usually weighted by market capitalisation. This means that companies with a higher market value have a bigger impact on the index's performance. Market capitalisation is calculated by multiplying the stock price by the total number of shares outstanding.
  • The index often represents various sectors of the economy, such as finance, healthcare, energy, and technology. This helps in providing a comprehensive view of the market's performance.
  • The composition of the index is not static. It is regularly reviewed and rebalanced, usually quarterly. During this process, companies can be added or removed from the index based on changes in their market capitalisation.
  • The index serves as a performance indicator for the segment of the market it represents. Investors and analysts use it to gauge market trends, compare investment returns, and assess economic health.

How is an index created?

Creating a stock index involves several steps and decisions to ensure it accurately represents the targeted segment of the stock market:

  • Define the objective: The first step is to define the purpose of the index. This includes deciding which part of the market it should represent, such as a specific sector, size of companies (large-cap, mid-cap, small-cap), or a particular style of investing (growth, value).
  • Based on the objective, a universe of stocks is identified. For a broad market index, this might include all stocks in a particular market or exchange. For a sector-specific index, it would include stocks within that sector.
  • Clear criteria are established for a stock to be included in the index. This often involves market capitalisation, liquidity (how easily stocks can be bought and sold), and financial health.
  • The index must decide how to weigh its components. The most common method is market capitalisation weighting, where larger companies have a greater impact on the index.
  • Indexes are not static and need rules for how and when they will be rebalanced. This might be done quarterly, semi-annually, or annually.
  • The index needs a calculation formula. This usually involves summing the weighted prices of its component stocks and then scaling it, often against a base value from a specific start date.
  • Regular review and maintenance are crucial.

Calculation methodology of stock indices

There is more than one way to calculate a stock index, but most have two elements in common:

  • All are measured in points that depend on the price of the shares that make up the index.
  • All have a start date.

In a price-weighted index, the index value is based on the price of the individual stocks. The Dow Jones Industrial Average (DJIA) is a famous example. Here, each stock's price is added together and then divided by a divisor. The divisor is adjusted for stock splits and dividends to maintain consistency.

Market capitalisation-weighted index is the most common methodology used for indices like the S&P 500 and the NASDAQ Composite. In this method, the market capitalisation of each company is calculated by multiplying the stock price by the total number of outstanding shares. The index is then the sum of these market caps, each weighted according to its proportion in the total market capitalisation of the index.

In an equal-weighted index, each stock contributes equally to the index, regardless of its price or market capitalisation. This means that the performance of smaller companies has a more significant impact on the index than in market cap-weighted indices.

Measuring market performance

Global indices serve as market benchmarks, providing essential inputs for products and investment strategies while yielding benefits by:

  1. Creating trading instruments.
  2. Generating valuable market analysis data.
  3. Serving as tools for research and decision-making.
  4. Promoting the trading of underlying assets within the index, both locally and internationally.

Benchmarks for investments and portfolio management

Understanding the significance of a benchmark index in investing is crucial.

A benchmark index serves as the initial reference point for portfolio managers when constructing portfolios. It sets the parameters for managing the portfolio, including the desired risk and return levels. Additionally, it allows investors to assess how their portfolios perform relative to the market.

Investors and portfolio managers commonly use stock indices as benchmarks to gauge investment performance and make asset allocation decisions. This is because stock indices provide insights into the general behaviour of specific asset classes.

For instance, if an investor achieves a 15% return in a year, that's generally considered a decent outcome. However, if the benchmark rose by 30% during the same year, their relative return falls significantly below the benchmark. In this case, the investor's return is not satisfactory, which means that they are underperforming the market.

Index Trading based on stock indices

Several financial instruments are based on indices, offering various ways for investors to gain exposure to specific market segments or the overall market. These instruments include:

  1. Index funds: These are mutual funds designed to replicate the performance of a specific index. They aim to match the index's return by holding the same stocks in the same proportions as the index. They are a popular choice for passive investment strategies. Check out our index fund guide.
  2. Exchange-traded funds (ETFs): ETFs are similar to index funds but are traded on stock exchanges like individual stocks. They track various indices and provide an easy way for investors to invest in broad market segments or specific sectors – have a look at our best ETFs to get started.
  3. Futures contracts: Index futures are agreements to buy or sell the value of an index at a specific price on a future date. Futures trading is used for hedging, speculation, or arbitraging differences between the index and the futures price.
  4. Options: Options trading gives the holder the right, but not the obligation, to buy or sell an index at a specified price before a certain date. They are used for strategies like hedging risk or speculating on the direction of the market.
  5. Derivative contracts: These include various types of derivatives like CFD trading, swaps, and structured products that are based on the performance of an index. They are used by sophisticated investors for complex investment strategies.

Economic indicators and forecasts

Stock indices also function as economic indicators because they provide insights into market movements within specific sectors or the broader market.

Indices serve as valuable guides, enabling the analysis and assessment of factors that measure market profitability and risk. They also allow for the replication of an index's performance in other contexts.


In summary, a UK stock index like the FTSE 100 index is a curated list of companies, weighted by market capitalisation, designed to provide a snapshot of the market's performance. It is regularly updated and serves as a tool for investors and analysts to understand and participate in the UK stock market. Index trading is based on investing on any of these indices.

In essence, stock indices provide a means to gauge the growth variations of companies within a country, an economic sector, and other relevant areas.


What are the most prominent UK stock indices?

Key UK stock indices include the FTSE 100 (top 100 companies), FTSE 250 (mid-cap companies), and the FTSE All-Share (all listed companies).

What do rising or falling stock indices indicate?

A rising index suggests overall market or sector growth, while a falling index may indicate a decline in market or sector performance.

Can stock indices be used for long-term investing?

Yes, stock indices can be used for long-term investing as they provide diversified exposure to a market or sector, making them suitable for investors with a long-term horizon.

Related Articles