An index in finance and investment is a statistical measure that tracks the performance of a group of assets or a stock market segment. It provides a snapshot of its overall health. It is widely used as a benchmark to compare the performance of specific investments or sectors. In the UK, prominent examples include the FTSE 100, which represents the 100 largest companies listed on the London Stock Exchange, and the FTSE 250, focusing on the next 250.

What is an index?

An index is a barometer for a particular market section, providing investors and analysts with an overview of market trends and patterns. It simplifies the vast, complex movements of the market into a single, understandable figure.

Think of an index as a dashboard in a car. Just as the dashboard shows you the speed and fuel level, an index gives you a quick view of how a specific part of the stock market is performing.

Types of indices in the UK market

  1. Market cap indices: Like the FTSE 100, these track companies based on market capitalisation. Essentially, this means by size from biggest to smallest.
  2. Sector-specific indices: Focused on specific industries, such as the FTSE TechMARK, highlighting just technology firms in the tech sector.
  3. Broad-market indices: Represent broader market trends, like the FTSE All-Share, which covers a wide range of stocks. It’s the big picture of the market that shows a bit of everything.

The role of indices in investment strategies

Indices are crucial for benchmarking the performance of investment portfolios. Investors can use them to assess a market segment’s health and guide strategic decisions. For instance, tracking the FTSE 100 can provide insights into the performance of large-cap stocks in the UK.

When the Bank of England changes interest rates, analysts will often discuss how this affects the FTSE indices, which in turn influences investment strategies.

Understanding index composition and calculation

Indices are typically weighted by market cap. A bigger market cap means a bigger company. They are recalculated regularly to reflect stock price changes, mergers, acquisitions, or company size shifts. 

For example, when a major company like Tesco or BP experiences significant stock price changes, it can impact the entire FTSE 100 due to their large size.

The impact of indices on market psychology

Indices not only reflect market performance but also influence investor psychology. Significant movements in key indices like the FTSE 100 can affect investor confidence and decision-making, leading to market-wide impacts.

During the 2020 stock market fall, caused in large part by the impact of the Covid-19 pandemic, the sharp fall in major indices like the FTSE 100 created a wave of panic among investors, with many seeking “safe shelters” in cash, money market funds or commodities like Gold.

How to use indices for diversified investment

Investors can use indices as a guide to diversify their portfolios. By understanding which sectors or companies are included in an index, investors can make informed decisions to spread their investments across different assets, reducing risk and tapping into other market segments.

It’s like using a recipe book to cook a balanced meal. Just as you choose different ingredients for nutritional balance, you might select various stocks across different indices for a balanced investment portfolio.

Recently, in response to the rise of sustainable investing, many investors will look to indices like the FTSE4Good Index, which includes companies meeting specific environmental, social, and governance criteria, to diversify their portfolios responsibly.


In the UK’s financial landscape, indices serve as essential tools for investors, offering insights into market trends and helping formulate investment strategies. Whether you are a seasoned investor or a newcomer, understanding and utilising indices is critical to navigating the stock market effectively.