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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11The S&P 500 and Nasdaq indexes move when the prices of their constituent stocks change, with the largest weighted companies usually having the greatest influence. Earnings expectations, interest rates, inflation, economic growth and investor risk appetite can all affect those stock prices. The exact answer depends on which Nasdaq index you mean: the Nasdaq Composite and Nasdaq-100 measure different groups of companies.
How stock prices turn into an index move
An index is a calculated measure of its constituents, not an independent force that moves the market. A stock’s influence depends on its index weight—not simply its share price. When a heavily weighted constituent rises or falls, it can move the index more than a smaller-weighted member.
Market-cap and float-adjusted weighting
In market-cap weighting, a company’s market value—its share price multiplied by its shares outstanding—determines its relative weight. Float-adjusted weighting excludes shares held in blocks that are not generally available for public trading, such as certain holdings by founders, executives, controlling shareholders, company foundations or governments. The S&P Dow Jones Indices explainer on index weighting describes these distinctions.
The S&P 500 is float-adjusted market-cap weighted. Its level is calculated by aggregating the market values of its constituents and scaling the result by a divisor. The divisor is adjusted for certain corporate actions and membership changes so that those events do not, by themselves, create artificial jumps in the index. See S&P DJI’s index mathematics methodology.
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What makes the S&P 500 go up or down?
The S&P 500 represents large U.S. companies, but it is not an equal-weighted list in which every company has the same effect. S&P Dow Jones Indices says it includes 500 leading companies and covers approximately 80% of available market capitalization. Those are descriptive figures from the provider’s 2026 profile, not a live measure of the market’s current composition. The index is rebalanced quarterly. S&P 500 profile.
Because weights differ, the largest constituents can account for a disproportionate share of a day’s move. The index may rise even when many members fall if a smaller number of high-weight stocks gain enough; the reverse can also happen. To explain a particular day or month, check dated constituent contributions rather than assuming that every stock—or one macroeconomic headline—moved the index in the same direction.
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What does “the Nasdaq” mean?
“Nasdaq” is ambiguous in everyday conversation. It may mean the Nasdaq Composite, a broad index of Nasdaq-listed companies, or the Nasdaq-100, a narrower index of large Nasdaq-listed non-financial companies. They have different eligibility rules and weighting methods, so their performance can diverge.
| Index | What it measures | Weighting |
|---|---|---|
| S&P 500 | 500 leading U.S. companies; S&P DJI says it covers approximately 80% of available market capitalization, according to its 2026 profile. | Float-adjusted market-cap weighted. |
| Nasdaq Composite | Nasdaq-listed companies across a broad range of sizes and sectors; Nasdaq says the index was established in 1971. | Market-cap weighted. |
| Nasdaq-100 | 100 of the largest Nasdaq-listed non-financial companies, according to Nasdaq’s methodology. | Modified market-cap weighted. |
Sources: S&P 500 profile, Nasdaq Composite overview and Nasdaq-100 methodology.
Nasdaq Composite
The Composite covers Nasdaq-listed companies across a broad range of sizes and sectors and is market-cap weighted. Nasdaq’s overview describes it as including thousands of stocks, with a particular emphasis on technology-related companies. It is not limited to technology firms, and it is not interchangeable with the Nasdaq-100. Nasdaq Composite overview.
Nasdaq-100
The Nasdaq-100 tracks 100 of the largest Nasdaq-listed non-financial companies and uses modified market-cap weighting. Nasdaq announced that updated methodology rules took effect on May 1, 2026. That means its composition and weights should be understood under the applicable methodology, not assumed to match the Composite’s. Nasdaq-100 methodology; Nasdaq methodology update announcement.
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How economic news affects index prices
Macroeconomic news affects an index indirectly when investors revise what they are willing to pay for its constituent stocks. Relevant channels include:
- Expected profits and cash flows: Stronger or weaker expectations for company sales, margins and future earnings can change share valuations.
- Interest rates and borrowing costs: Changes in financing costs can affect companies’ expenses, while changes in discount rates can alter how investors value expected future cash flows.
- Inflation and economic activity: These can influence costs, demand, company outlooks and expectations for monetary policy.
- Risk appetite: Investors’ willingness to hold riskier assets can shift demand for stocks and affect valuations.
These are mechanisms, not a ranking of what is driving returns right now. Index calculation and methodology documents explain how an index is composed and measured; they do not establish which economic factor currently explains its daily or monthly performance. A claim about the current driver needs a defined time period and evidence tied to that period.
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Why index rules and return types matter
Prices are only part of what defines an index. Eligibility criteria decide which companies can enter; selection, weighting and rebalancing rules determine how much each member counts and when the index changes. For the S&P 500, the provider identifies quarterly rebalancing. For the Nasdaq-100, consult the methodology effective May 1, 2026, when interpreting its rules. Membership and weights can change, so use dated information when comparing current concentration.
Also check which return series a chart or performance figure uses. A price-return index reflects share-price changes. A total-return series also incorporates dividend income reinvestment. The two can produce different performance figures over the same period; S&P DJI explains the calculation in its index mathematics methodology.
What to check when comparing the S&P 500 with a Nasdaq index
- Name the benchmark: Specify Nasdaq Composite or Nasdaq-100 rather than saying only “Nasdaq.”
- Compare the universe: The S&P 500 represents leading U.S. companies; the Composite covers Nasdaq-listed companies broadly; the Nasdaq-100 covers large Nasdaq-listed non-financial companies.
- Check weighting and concentration: Weighting rules determine how much large constituents can sway an index. Use dated constituent and sector weights for a current concentration comparison.
- Check methodology dates: Eligibility, rebalancing and reconstitution rules matter, and methodology updates can change how an index is maintained.
- Match return types: Compare price return with price return, or total return with total return, over the same dates.
An index level is a benchmark calculation, not a company share price or an investment return by itself. An index-linked fund seeks exposure to a particular benchmark, but the fund is a separate product; its objective and tracking approach should be checked rather than treating ownership of a fund as identical to owning the index.
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