A fictional five-company market
Suppose Company A begins a session with 60% of a simplified index's weight. Companies B, C, D and E each have 10%. A rises 4%; each of the others falls 2%. Assume a fixed starting basket and no corporate actions, dividends or changes in index membership during the session.
| Company | Starting weight | Stock return | Index contribution |
|---|---|---|---|
| Company A | 60% | +4.0% | 2.4 percentage points |
| Company B | 10% | −2.0% | -0.2 percentage points |
| Company C | 10% | −2.0% | -0.2 percentage points |
| Company D | 10% | −2.0% | -0.2 percentage points |
| Company E | 10% | −2.0% | -0.2 percentage points |
Weighted basket: +1.6% · Equal-weight starting basket: −0.8% · Advancing stocks: 1 of 5
A contributes 60% × 4% = 2.4 percentage points to the index return. Each other company contributes 10% × −2% = −0.2 percentage points. Together, the four decliners subtract 0.8 points. The index gains 1.6%, even though four of five stocks fall.
The simple average stock return is different: (4% − 2% − 2% − 2% − 2%) ÷ 5 = −0.8%. That is the return of this basket if it starts with equal weights. Both calculations are correct for their stated weighting rules.
What the headline is actually saying
A positive index return tells you that the weighted basket rose. It does not tell you that a majority of constituents rose or that a randomly selected portfolio had a positive session. Those are additional questions.
In this example, the headline gain depends heavily on A. Describing leadership as concentrated is an observation about contributions. It does not establish that A is overvalued, that the index is manipulated or that a decline is imminent.
Use a comparison that holds the question steady
To examine weighting, compare baskets with the same constituents over the same period and with compatible return treatment. Comparing a five-stock basket with a different country's index would change more than the weights.
Real indexes follow detailed rules for eligible shares, corporate actions, membership and rebalancing. This example isolates one arithmetic mechanism. It is not a replica of a named index, a real day's data or a backtest.
Three questions to take into the next market report
- Which index is the headline describing, and how is it weighted?
- How many constituents advanced or declined over that same session?
- Did the largest contributors dominate, or did gains extend across the basket?
A report becomes easier to interpret when you separate the headline, participation and the explanation. You can understand why a market rose without treating that explanation as a forecast for the next session.
If A's gain were only 1% while the other four stocks still fell 2%, what would this weighted index return?
Show the explanation
A would contribute +0.6 percentage points and the others −0.8 points combined. The index would fall 0.2%. The same one-winner/four-loser count can accompany a different index direction.
Sources and editorial notes
Original explanations and hypothetical examples prepared with AI assistance. Primary references checked October 16, 2026. Numerical examples are hypothetical unless explicitly identified otherwise.
- SEC Investor.gov: Market Index
An index measures the performance of a selected basket of stocks representing a market or sector.
- SEC Investor.gov: Market Capitalization
Market capitalization is share price multiplied by total outstanding shares.
- S&P Dow Jones Indices: S&P 500 Equal Weight Index
Same constituent companies as the capitalization-weighted S&P 500; equal company weights reset at quarterly rebalances.