How do market indices help investors understand market performance?

A market index is an investment tool designed for measuring performance, not investing. The first step for any index is to select the securities to be included in the sample. Next, a weighting scheme needs to be established for each of the included securities. Finally, indices must be updated on a regular basis to measure fair market values.

Many people use indices to build portfolios of shares, to evaluate the performance of fund managers, and to report to committees. In these cases, it is less important to know whether or not the index went up or down, and more important to know what such movements mean, and what performance they measure.

What the number is actually measuring

An index level is worthless by itself but very useful in comparison to the index’s history and/or other indexes. A broad market index like the Dow Jones Industrial Average, S&P 500, or Hang Seng Index consists of hundreds of large capital domestic companies. Such an index is therefore worth little as a representation of smaller listed companies which are influenced by different factors such as domestic demand, credit, and refinancing.

Breadth helps to distinguish between a healthy advance and a narrow advance. Thus, if the Index has gone up 3% and the median constituent is flat, the 3% gain must have been due to a handful of constituents. We compare a Capitalization Weighted Index with an Equal Weighted Index of the same constituents to check for this.

Price return versus total return

Just about all reported index quotes are expressed in terms of a price return (i.e. Do not include dividends). Multi-year historical data as well as current performance data of investment managers should all be expressed in terms of a total return. Only when a very large portion of a long-term return of a market comes in the form of dividends would one report a price return instead of total return, to report a ‘fair’ return for a fund that reinvests the dividends of a reported price return series.

How weighting methods change the story

Weighting can have the largest impact of any factor when comparing two or more market indices, often producing vastly different results even when monitoring relatively similar stocks. Each approach embodies a specific investor opinion regarding the optimal representation of market stocks.

Method	Weight driven by	Structural bias	Best used for
Free-float capitalization	Investable market value	Toward large, recently appreciated firms	Representing the aggregate investor experience
Equal weight	Fixed allocation per constituent	Toward smaller constituents, higher turnover	Measuring breadth and average stock behavior
Price weight	Share price alone	Distorted by nominal price, splits	Historical continuity only
Fundamental weight	Revenue, earnings, book value	Toward value characteristics	Benchmarking value-oriented mandates

Weighting, Capitalization weighting has one clear property, All investors can hold the same market simultaneously. This is why the capitalization-weighted index is the only ‘neutral’ index. It is also worth noting that a concentrated market leads to a concentrated benchmark. Moreover, most passive investment mandates unintentionally concentrate exposure.

Reading sector and regional indices against the whole

Sector and industry indices are better used as relative measures of performance, e.g. A sub-industry could be rising 8% when the broader market is up 7% whereas on the other hand it could be up 8% on a flat market. Relative strength lines are also useful and are calculated by dividing a sector index by that of its parent, such as a sub-industry index divided by the industry index. This will remove the ‘common factor’ in performance and highlight what is truly unique to that sub-industry.

Watch the classification boundaries

When sectors are defined by revenue-based classification schemes, there can be a lag to changes in a company’s business model. A subscription software company, for example, might be classified as a consumer or industrial company. Look at the top stocks by weight in the sector index, and review a practical guide to how a stock market index is built and traded, to form a more accurate picture of what is happening in the space.

Where indices support diversification decisions and where they mislead

Indices are also the material for all kinds of correlation, factor exposure and risk building analysis. And they generate many common errors.

• Survivorship in long histories. Constituents that failed were removed, so back-tested series flatter the strategies tested against them.
• Reconstitution drag. Scheduled rebalancing is publicly known, and predictable trading around those dates affects returns that real portfolios must absorb.
• Currency framing. An international index quoted in local currency describes an experience no unhedged foreign investor had.
• Apparent diversification. Holding several regional indices can still concentrate exposure if the same global mega-caps dominate each one.

A workable checklist before adopting a benchmark

1. Confirm the eligibility rules match the mandate’s investable universe, including liquidity and listing requirements.
2. Verify the return basis, currency, and any hedging assumption in the published series.
3. Review the largest ten weights and the effective number of constituents.
4. Check reconstitution frequency and whether buffer rules reduce turnover.
5. Document the fallback if the provider changes methodology mid-mandate.

Using indices as a working instrument

Treat the index as a measurement with known errors. Multiply the headline index by a breadth index, by a volatility index, and by at least one other index that uses a different weighting method. The biggest differences are usually most meaningful.