A stock market index is a tool for measuring
the performance of an entire stock
market or group of related stocks. These indices are often associated
with particular stock exchanges or industries. They exist because
changes in a market index can reflect a more general price trend
than a change in individual stock prices.
There has been an accelerating trend in recent decades to create
investment funds that are based on market indices. These are called
The most regularly quoted market indices are those including the
stocks of the largest listed companies on a nation's largest stock
exchange. Examples include the American Dow Jones Industrial Average,
the S&P 500, the British FTSE 100, and the Japanese Nikkei 225.
However, because of the concentration on a relatively few very large
stocks, such indices can give a misleading picture of market trends.
More specialised indices exist tracking the performance of specific
sectors of the market - for instance, there are national indices
for mining companies, or companies that mine a specific commodity,
small companies, technology companies, and even very specialised
indices such as one by Linux Weekly News tracking stocks of companies
that sell products and services based on Linux.
A notable specialised index type is those for ethical investing
indices that include only those companies satisfying ecological
or social criteria, e.g. those of The Calvert Group, Domini, and
the Dow Jones Sustainable Index.
Another important trend is strict mechanical criteria for inclusion
and exclusion to prevent market manipulation, e.g. as in Canada
when Nortel was permitted to rise to over 50% of the TSE 300 index
value. Ethical indices have a particular interest in mechanical
criteria, seeking to avoid accusations of ideological bias in selection,
and have pioneered techniques for inclusion and exclusion of stocks
based on complex criteria. Another means of mechanical selection
is mark-to-future methods that exploit scenarios produced by multiple
analysts weighted according to probability, to determine which stocks
have become too risky to hold in the index of concern.
Critics of such initiatives argue that many firms satisfy mechanical
"ethical criteria", e.g. regarding board composition or hiring practices,
but fail to perform ethically with respect to shareholders, e.g.
Enron. Indeed, the seeming "seal of approval" of an ethical index
may put investors more at ease, enabling scams. One response to
these criticisms is that trust in the corporate management, index
criteria, fund or index manager, and securities regulator, can never
be replaced by mechanical means, so "market transparency" and "disclosure"
are the only long-term-effective paths to fair markets.
The Dow Jones Industrial Average is not an index, because it is
price-weighted and not market-capitalization weighted. Technically,
an index must be market-cap weighted. It is impossible to buy-and-hold
a collection of stocks that give you the same return as the Dow,
because it is price-weighted.
Historically, the Dow is price-weighted because that is the easiest
way to compute a composite. With computers, it is possible to create
a market cap-weighted composite, which can be mirrored by an investment