Index

What is an index?

An index is a number that summarizes a group of items. In finance it usually refers to a collection of securities, like stocks or bonds, that represent part of a market. The index gives one number you can watch. That number tells you if the group is going up or down.

Think of an index as a thermometer. The thermometer does not show every particle in the air. It shows a single reading that reflects the whole room. A stock index does the same for a market or a sector.

Common examples

• S&P 500: 500 large U.S. companies. It is a market-cap weighted index.
• Dow Jones Industrial Average: 30 large U.S. companies. It is price-weighted.
• Nasdaq Composite: Many technology and growth companies listed on Nasdaq.
• FTSE 100, Nikkei 225: Major indexes for the UK and Japan respectively.
• Bond and commodity indexes: Track bonds or physical commodities like oil and gold.

Why indexes matter

Indexes are useful because they:

• Give a quick view of market performance.
• Serve as benchmarks for managers. Fund managers compare their returns to an index.
• Allow passive investing. Index funds and ETFs aim to match an index.
• Help researchers and analysts study markets.

How an index is built

Three common ways to weight components:

1. Market-cap weighted

   • Each company is weighted by its market value. Larger companies move the index more.
   • Example: Company A has market cap 60, Company B 40. A move of 10% in A affects the index more than a 10% move in B.
   • Simple rule: index return roughly equals the weighted average of component returns.

2. Price-weighted

   • Each company is weighted by its share price, not its size.
   • If a high-priced stock moves, it moves the index more.
   • The Dow Jones is the classic price-weighted index.

3. Equal-weighted

   • Every company has the same weight regardless of size.
   • This gives smaller companies more influence than in a market-cap index.

Indexes also have rules about which companies get included. They set criteria for size, trading volume, and sector.

How index levels are calculated

An index level is not a raw sum of stock prices or values. Often a divisor or base value is used to make the index easier to read and compare over time.

• Market-cap weighted indices use the total market cap of components divided by a divisor.
• Price-weighted indices add prices and divide by a divisor to handle stock splits and corporate actions.

You do not need to know the exact divisor to use an index. The main point is this: index moves reflect the weighted moves of the parts.

Example for clarity:

• Two stocks, A and B. A is $60 billion market cap, B is $40 billion.
• Weight A = 60/(60+40) = 0.6. Weight B = 0.4.
• If A gains 5% and B gains 1%, the index gain ≈ 0.65% + 0.41% = 3.4%.

How investors use indexes

You cannot buy an index directly. You can buy products that track them:

• Index funds and ETFs hold a portfolio designed to match the index.
• Some funds fully replicate the index. Others sample it.
• Index tracking is a low-cost way to get broad market exposure.

Indexes also form the basis for derivatives like futures and options.

Limitations and pitfalls

• An index is not the whole market. It reflects what it is designed to cover.
• Market-cap indexes give big companies more power. That can lead to concentration risk.
• Price-weighted indexes can be skewed by one high-priced stock.
• Survivorship bias matters. Old components that failed may be removed, making past performance look better.
• Index rules can change. Companies may be added or removed, which shifts the index.

When to care

You should care about indexes when you:

• Compare investment performance.
• Choose an index fund or ETF.
• Read market news that cites index moves.

If you want to track a part of the market, pick an index that matches that part. For U.S. large caps use S&P 500. For technology stocks use Nasdaq or a tech sector index. For small companies use a small-cap index.

Takeaway

An index is a simple, single number that represents a group of securities. It is a tool for measurement and comparison. Knowing how an index is weighted and what it covers helps you understand what its moves mean. If you want to invest in an index, use an index fund or ETF and mind the index rules and risks.