Learn What Market Indexes Say About Investing

Reflection of stock market graph in window
••• Hiroshi Watanabe/ Taxi Japan/ Getty Images

The Dow is up (or down) so the market is great (or terrible). If you read or listen to the popular media, you might get the impression that the Dow Jones Industrial Average, usually just the Dow, is the pulse of the market. Other stock indexes like the S&P 500 or the Nasdaq Composite also get mentioned, often in hushed or excited tones depending on numbers. However, what do these and the other reported indexes really tell us and how should you use them?

Index Numbers

First, take a look at what an index number represents. Although there are different ways to calculate index numbers, it is important to remember the numbers represent a change from an original or base value.

The number is not important. What is important is the percent change over time. This movement up or down gives you an idea of how the index is performing. Is the Dow up or down? The index is calculated “on the fly” during trading to give investors a sense of direction to the market it represents.

Notice, I said the index reflects “the market it represents,” not “the market.” Most stock indexes, even those quoted as representing the total market, only reflect a portion of the actual market. Here are the most popular indexes and the markets they reflect.

The Dow

The Dow Jones Industrial Average is the oldest and most widely known index. It is also the most widely quoted index and, mistakenly, considered the market barometer.

Originally, it was a simple average of the stocks in the index, but thanks to​ stock splits, spin-offs, and other transactions, more sophistication is now required. You can find more information at site. The Dow currently has only 30 stocks. However, each of these stocks represents one of the most influential companies in the U.S.

The Dow is the only major index that is price-weighted, which means if a stock’s price changes by $1, it has the same effect on the index regardless of the percent change for the stock. In other words, a $1 change for a $30 stock has the same effect as a $1 change for a $60 stock.

The calculation of the Dow takes into account numerous stock splits over the years. By adjusting the math, it is possible to keep a historically viable index meaningful. The Dow stocks represent about one-quarter of the value of the total market, so in that sense, it is a factor and big changes indicate investor confidence in stocks, however, it does not represent small or mid-size companies at all.

S&P 500

The S&P 500 is the most frequently used index by financial professionals as a representative of “the market.” It includes 500 of the most widely traded stocks and leans towards the larger companies.

It covers about 70 percent of the market’s total value, so in those terms, it is much closer to representing the true market than the Dow. The S&P 500 is a market capitalization or market cap weighted index, as are almost all of the major indexes.

Weighting by market cap gives more importance to larger companies, so changes in Microsoft stock will have a greater impact than almost any other stock in the index. Even though the S&P 500 is weighted toward larger companies, it is a more accurate gauge of the broader market than the Dow is.

Even though some of the talking heads on TV may emphasize the Dow, you will get a clearer picture of the market by focusing your attention on the S&P 500.

The Nasdaq Stock Market Composite

The Nasdaq Stock Market Composite is composed of all the stocks on the Nasdaq market – more than 5,000. Although broad in coverage, the Nasdaq is heavily weighted to technology stocks. This is because it is a market cap weighted index and stocks like Microsoft and some of the other big technology companies influence the index.

Their influence and the population of small, speculative companies in the Nasdaq make the index more volatile than either the Dow or the S&P 500. The Nasdaq obviously is not designed to represent “the market,” however it does give you a good idea of where technology investors are going.

Other Indexes

There are a number of other indexes that measure larger or smaller sections of the market. Mutual fund investors can find a number of funds that track almost any index they want.

However, the major three indexes above will serve most investors well. Should you want to look at other indexes for comparison, make sure you understand how the index is weighted (most, if not all, will be a market cap) and how stocks are selected.

What’s Good about Indexes

Indexes provide useful information including:

  • Even with their limitations, indexes show trends and changes in investing patterns.
  • They give us snapshots, even if they are out of focus.
  • Indexes provide a yardstick for comparison.

What’s Wrong with Indexes

Indexes, by design, have major flaws that make them suspect as truly representative of much of anything.

  • People decide which stocks to include and which to remove and people make mistakes. So, sometimes stocks are included that shouldn’t be and stocks are removed that shouldn’t be; and so on. In addition, this process repeats year after year, so it is hard to look back and compare the S&P 500 of 1995 with the S&P 500 of 2004.
  • By weighting the indexes (except for the Dow) by size, disproportion representation goes to large or giant companies. If one of them has a bad day, it can shake the whole index.

    What About Indexes?

    There are a few things investors need to remember about the indexes:

    • Indexes are not the market. No matter what the big three indexes say, you should stay focused on your stocks or targets for evaluation. Pick any day that all three indexes are down and I can almost guarantee that there will be stocks setting new highs the same day.
    • Indexes react to actual trades. If you listen to some of the TV talking heads, you might think the indexes move on emotion. Investors may trade on the expectation of good or bad news, but indexes are mathematical calculations, not tea leaves.
    • Focusing on day by day, hour by hour, minute by minute clicks of an index is a good way to waste time.
    • Indexes provide a better historical perspective than forecasting service. They can be especially helpful when viewed over a long period in spotting trends.