7.2 Security Market Indexes and Efficiency
Key Takeaways
- Security indexes measure the performance of a defined market, segment, strategy, or asset class.
- Price-weighted, equal-weighted, and value-weighted indexes respond differently to price changes and constituent size.
- Index construction choices include the target market, selection rules, weighting method, rebalancing, and reconstitution.
- Price return indexes exclude income, while total return indexes include income reinvestment.
- Market efficiency describes how quickly and accurately prices reflect information, not whether prices are always correct.
What an index measures
A security market index is a portfolio designed to represent a market or segment. It can track large-cap stocks, small-cap stocks, a country, a sector, a style, a factor, or a custom strategy. Indexes are used as benchmarks, performance attribution tools, proxies for asset classes, and bases for index funds and derivatives.
The first decision is the target market. The second is which securities qualify. The third is how each security is weighted. The fourth is how the index is maintained when prices move, shares outstanding change, mergers occur, or companies enter and leave the target universe.
Weighting methods
A price-weighted index weights each constituent by its share price. A high-priced stock has more influence than a low-priced stock, even if the lower-priced company has a much larger market value. Stock splits require divisor adjustments so the index level is not mechanically distorted.
An equal-weighted index gives each constituent the same weight at each rebalance date. It gives smaller companies more influence than a value-weighted index would. Equal weighting requires periodic rebalancing because price changes cause weights to drift away from equality.
A value-weighted index weights each stock by market capitalization. A float-adjusted value-weighted index uses only shares available to public investors. This method is common because it reflects the aggregate value investors can hold and requires less rebalancing after ordinary price moves.
Return measurement and maintenance
A price return index reflects only price changes. A total return index includes dividends or interest as if reinvested. For equities, the difference can be material over time. If a question includes dividend income, total return is the relevant concept.
Rebalancing resets weights to the target method. Reconstitution changes the membership list. A style index may rebalance weights quarterly and reconstitute membership annually. These maintenance rules can create turnover, trading costs, and timing effects for funds tracking the index.
Structured aid: index construction checklist
| Question | Candidate action |
|---|---|
| What market is represented? | Identify the target universe |
| Which securities are included? | Apply selection and eligibility rules |
| How are securities weighted? | Price, equal, value, float, or factor weight |
| How is return reported? | Separate price return from total return |
| How is the index maintained? | Distinguish rebalancing from reconstitution |
Uses and limitations
Indexes are convenient, but they are not neutral facts. A benchmark can be too broad, too narrow, or mismatched to a manager's mandate. Survivorship bias can appear if failed or delisted firms are excluded from a historical sample. Concentration can make a value-weighted index depend heavily on a few large companies.
Free-float adjustment matters because shares held by governments, founders, or strategic owners may be unavailable for trading. A market-cap index based on total shares may overstate investable opportunity. A float-adjusted index better reflects the securities available to ordinary investors.
Market efficiency
An informationally efficient market reflects available information quickly and rationally enough that investors cannot consistently earn abnormal risk-adjusted returns after costs. Efficiency is a statement about competition, information, and pricing, not a claim that every price equals intrinsic value at every moment.
Weak-form efficiency means prices reflect past market data such as prices and trading volume. If weak form holds, technical analysis based only on historical price patterns should not consistently produce abnormal returns after costs.
Semi-strong form efficiency means prices reflect all publicly available information. If semi-strong form holds, fundamental analysis of public financial statements and news should not consistently earn abnormal returns after costs. Strong-form efficiency means prices reflect public and private information. This is the strictest and least realistic form.
Inefficiency and behavioral traps
Market anomalies, limits to arbitrage, transaction costs, short-sale constraints, taxes, and investor behavior can weaken efficiency. Behavioral biases such as overconfidence, herding, anchoring, and loss aversion can affect prices. The exam usually wants a balanced answer: inefficiencies may exist, but exploiting them is costly and uncertain.
Exam focus
For index questions, identify the weighting method before doing arithmetic. A price-weighted index reacts most to the highest share price. A value-weighted index reacts most to the largest market value. An equal-weighted index gives every constituent the same starting influence.
For efficiency questions, match the information set. Past prices point to weak form. Public financial statements and news point to semi-strong form. Private insider information points to strong form. Abnormal returns must be risk-adjusted and considered after trading costs.
In a price-weighted equity index, the constituent with the greatest influence on the index return is the stock with the highest:
An index that includes dividend reinvestment in its reported performance is best described as a:
If stock prices fully reflect all publicly available information, the market is best described as: