Direct Answer
US equity indexes are not interchangeable measures of one market. They differ in which companies are eligible, how those companies are selected, and how much each one contributes, so two indexes covering what sounds like the same segment can hold different companies in different proportions and diverge for years.
The major US indexes side by side
Target count is shown only where the number is definitional, meaning it appears in the index's own name. A target is not the exact membership on any given day: corporate actions, dual share classes and pending changes move the real number around it.
| Index | Provider | What it covers | Target count |
|---|---|---|---|
| S&P 500 | S&P Dow Jones Indices | Large-cap U.S. equities | 500 |
| Dow Jones Industrial Average | S&P Dow Jones Indices | 30 U.S. blue-chip companies; price weighted | Not fixed by the name |
| Nasdaq Composite | Nasdaq | Nasdaq-listed common-type stocks | Not fixed by the name |
| Nasdaq-100 | Nasdaq | 100 largest non-financial Nasdaq-listed companies | 100 |
| Russell 2000 | FTSE Russell | U.S. small-cap segment | 2000 |
| Russell 1000 | FTSE Russell | U.S. large-cap segment | 1000 |
| Russell 3000 | FTSE Russell | Broad investable U.S. equity market | 3000 |
| S&P MidCap 400 | S&P Dow Jones Indices | U.S. mid-cap equities | 400 |
| S&P SmallCap 600 | S&P Dow Jones Indices | U.S. small-cap equities | 600 |
| S&P Composite 1500 | S&P Dow Jones Indices | Combined S&P 500/400/600 | 1500 |
| S&P Total Market Index | S&P Dow Jones Indices | Broad U.S. equities | Not fixed by the name |
| Dow Jones U.S. Total Stock Market Index | S&P Dow Jones Indices | Broad U.S. equities | Not fixed by the name |
| Nasdaq Next Generation 100 | Nasdaq | Next 100 largest non-financial Nasdaq companies outside NDX | 100 |
| Nasdaq Financial-100 | Nasdaq | Large Nasdaq-listed financial companies | 100 |
| PHLX Semiconductor Sector Index | Nasdaq | Semiconductor industry benchmark | Not fixed by the name |
| Nasdaq Biotechnology Index | Nasdaq | Nasdaq-listed biotechnology/pharma benchmark | Not fixed by the name |
How to read a comparison like this
Two columns do most of the work. Coverage tells you which slice of the market is being measured, and the target count tells you how concentrated that measurement can get: an index of thirty companies and an index of three thousand answer very different questions even when both are described as measuring US stocks.
What the table deliberately does not carry is index levels, returns or constituent weights. Levels are not comparable across indexes because each has its own base value and formula, and constituent data is licensed by the providers. Each index page links to its provider for current membership.
Frequently Asked Questions
Which US index is the best measure of the market?
There is no single answer, because they measure different things. A large-cap index says nothing about small companies, a total-market index dilutes the largest ones, and a price-weighted index answers a different question again. The useful question is which segment you want measured.
Why do two indexes covering the same segment disagree?
Eligibility rules and weighting. Two providers can define "large cap" by different thresholds, apply different liquidity screens, and weight the result differently, so their indexes hold different companies in different proportions.
Can I compare index levels directly?
No. A level is the output of one index's own formula and base value, so the number itself is not comparable across indexes. Percentage change over the same period is comparable; the level is not.