A stock market index is a curated, rules-based sample of companies designed to represent the performance of a market, a sector, or a specific investment style. Rather than tracking every listed share, which would be impractical and noisy, an index acts as a manageable benchmark. For a long-term investor in Europe, whether based in Romania or elsewhere, indices provide a shared language for describing market movements and a foundation for building diversified portfolios through funds.
Understanding how indices are constructed, weighted, and rebalanced helps you interpret headlines, choose appropriate benchmarks for your portfolio, and anticipate the behaviour of the funds that track them. This guide explains the core concepts from first principles, using concrete examples and the numbers behind the most widely followed indices.
What Is a Stock Market Index?
At its simplest, an index is a basket of stocks chosen according to a published set of rules. The basket is not random; it is designed to reflect the composition of a broader market or a particular segment within it. For example, an index might aim to capture the largest companies in a country, the most liquid shares on a specific exchange, or firms from a single industry such as technology or healthcare.
The value of an index is calculated from the prices (or market capitalisations) of its constituent companies, and it moves as those prices move. Investors use indices in three main ways. First, as a benchmark to judge whether a portfolio or fund manager has performed well. Second, as a proxy for an entire market when constructing diversified portfolios. Third, as the underlying asset for index-linked products such as ETFs, futures, and options.
Because an index is a sample, its composition matters. A well-designed index balances representativeness with practical constraints such as liquidity and transparency, ensuring that it faithfully mirrors the market it claims to measure.
How Indices Are Built: Selection Criteria and Weighting
Index providers follow a systematic process to decide which companies enter the basket and how much influence each company has on the index's movements.
Selection criteria
Companies are typically chosen based on a combination of quantitative and qualitative filters. Common quantitative criteria include market capitalisation, which reflects the total market value of a company's shares; free float, the portion of shares actually available for public trading; and liquidity, measured by trading volume or the bid–ask spread, which ensures that the stocks can be bought and sold efficiently. Sector representation rules may also be applied to prevent the index from becoming overly concentrated in a single industry.
For example, the S&P 500 uses a committee-driven selection process and aims to capture approximately 83% of the total market capitalisation of US public companies. The BET, Romania's main domestic index, tracks the most liquid stocks on the Bucharest Stock Exchange, with the number of constituents varying between 10 and 20 companies and a single-company weight capped at 20% of total index capitalisation.
Weighting methods
Once companies are selected, index providers must decide how much weight each company receives. The three most common approaches are capitalisation-weighted, price-weighted, and equal-weighted.
In a capitalisation-weighted index, each company's influence is proportional to its market capitalisation (adjusted for free float). A large company like Apple therefore moves the index more than a small-cap firm. Most major global indices, including the S&P 500 and the MSCI World, use this approach.
A price-weighted index gives greater influence to companies with higher share prices, regardless of their overall size. The Dow Jones Industrial Average is the most famous example: it is a price-weighted measure of 30 US blue-chip companies, meaning a $300 stock moves the index more than a $30 stock, even if the smaller company has a larger market capitalisation.
An equal-weighted index assigns the same influence to every constituent, so a small company has exactly the same impact as a large one. This approach requires frequent rebalancing to maintain equal weights as share prices diverge.
| Index | Weighting method | Number of constituents |
|---|---|---|
| S&P 500 | Free-float capitalisation-weighted | 500 |
| MSCI World | Free-float capitalisation-weighted | 1280 |
| Dow Jones Industrial Average | Price-weighted | 30 |
| BET | Free-float capitalisation-weighted | 10–20 |
Free Float Explained
Free float refers to the shares of a company that are readily available for trading by the public. It excludes shares held by governments, founders, controlling shareholders, and certain locked-in holdings. When an index uses float-adjusted market capitalisation, it multiplies the share price by the number of freely tradable shares, not the total number of shares outstanding.
This distinction matters because a company may have a large total market capitalisation but only a small fraction of its shares actually trade on the open market. Weighting by free float prevents a company with many non-tradable shares from dominating the index in a way that does not reflect investable market reality. The S&P 500 adopted float-adjusted capitalisation weighting in 2005, and the BET has used a free-float methodology since its inception.
The S&P 500: Barometer of the US Market
The S&P 500 was launched in its current form on March 4, 1957, and has since become the most widely referenced gauge of the US equity market. It tracks the stock performance of 500 large-cap companies listed on US stock exchanges and includes approximately 83% of the total market capitalisation of US public companies.
The index is maintained by a committee that reviews constituents periodically, ensuring the sample remains representative of the evolving US economy. In 2005, the S&P 500 transitioned to public float-adjusted capitalisation weighting, aligning its methodology with global best practice.
For investors, the S&P 500 serves multiple roles. It is a benchmark for US equity performance, a component of economic indicators such as the Conference Board Leading Economic Index, and the foundation for a vast ecosystem of funds and derivatives. Approximately $13 trillion in assets were tracking the S&P 500 as of December 31, 2024, and the SPDR S&P 500 ETF Trust alone holds roughly $800 billion in assets. Since its inception in 1926, the S&P 500's compound annual growth rate including dividends has been approximately 9.8%, or 6% after inflation.
Going Global: The MSCI World
While national indices capture domestic markets, global indices allow investors to gain exposure to international equity markets within a single benchmark. The MSCI World Index is one of the most prominent examples. It captures large and mid-cap representation across 23 Developed Markets countries and covers approximately 85% of the free-float-adjusted market capitalisation in each country.
The MSCI World currently has 1,280 constituents with an index market capitalisation of $91.70 trillion (data as of August 31, 2026). The number of constituents has historically been around 1,500, reflecting the inclusion of large and mid-cap stocks across the developed world. For a European investor, the MSCI World provides a diversified starting point that spans the US, Europe, Japan, Australia, and other developed economies, reducing single-country risk.
National Indices: The BET Example
Local indices serve a specific purpose for investors focused on a single country's market. The BET (Bucharest Exchange Trading) Index, introduced on 19 September 1997 with an initial value of 1,000 points, tracks the most liquid stocks on the Bucharest Stock Exchange. It started with ten companies and a total market capitalisation of 443 million Romanian lei.
The BET is a free-float capitalisation-weighted price index. Its constituents have varied between a minimum of 10 and a maximum of 20 companies, and any single company's weight is capped at 20% of total index capitalisation. In 2015, the methodology was enhanced to include qualitative criteria related to transparency, reporting quality, and investor communication, in addition to quantitative liquidity metrics. For a Romanian investor, the BET provides a clear benchmark for domestic equity performance and a practical reference point when evaluating local funds or deciding which shares to research further.
Rebalancing and Its Effects on Investors
Indices are not static. Providers periodically review and adjust their composition to ensure they continue to meet their stated objectives. The S&P 500 rebalances quarterly, in March, June, September, and December, and most major stock indices rebalance quarterly or semi-annually.
The mechanics of rebalancing
When a company is added to or removed from an index, ETF providers and other passive funds that track the index must buy shares of newly added companies and sell those being removed. This forced trading can create temporary price distortions.
The index effect
The index effect describes how passive strategies tend to buy stocks at relative price peaks and sell at relative lows during index changes. Because additions are often announced in advance and attract buying interest, newly added stocks may already have risen by the time they are included. Conversely, stocks being removed may face selling pressure, pushing their prices down. These distortions are usually short-lived but can be material for the affected shares.
Tracking error
Tracking error — the divergence between a fund's returns and its benchmark — is influenced by factors including rebalancing strategies, transaction costs, and market volatility. Even a well-managed index fund will not perfectly replicate its benchmark, and investors should expect some degree of tracking error as a normal feature of passive investing.
Key Takeaways
- A stock market index is a curated, rules-based sample of companies that represents a market or segment, allowing investors to benchmark performance and build diversified portfolios.
- Indices are built using selection criteria such as market capitalisation, free float, and liquidity, and weighted by capitalisation, price, or equal weight depending on the methodology.
- Free-float adjustment ensures that only shares available for public trading influence the index, reflecting investable market reality.
- The S&P 500 is the dominant US benchmark, the MSCI World provides global developed-market exposure, and national indices like the BET offer a starting point for domestic investors.
- Rebalancing drives forced buying and selling by passive funds, creating the index effect and temporary tracking error that investors should understand when using index-linked products.
Educational only — not personalised investment advice.