What Is Weighted Index Contribution?

The simplified relationship

For a capitalization-weighted index, a useful small-move approximation is constituent weight multiplied by constituent return. Expressed in percentage points: contribution ≈ weight × return, with the weight treated as a fraction.

For example, a 10% index weight and a +2% stock return imply about +0.20 percentage points of index return. A 2% weight and a +5% return imply about +0.10 percentage points. The first stock moved less by percentage, but its approximate contribution was larger.

From percentage points to index points

A simplified index-point estimate multiplies the percentage-point contribution by the chosen index level and divides by 100. If the estimated contribution is +0.20 percentage points and the index level is 6,000, the corresponding arithmetic estimate is +12 index points.

This is a teaching calculation. Official index returns depend on the index provider's full methodology, reference prices, share counts, divisor, corporate actions, and timing.

Signs and baskets matter

Positive and negative contributions offset one another. Summing only a selected group gives that basket's partial contribution, not the entire benchmark's daily return. Missing constituents and stale or differently timed data can change the comparison.

How the app makes the comparison visible

Market Watch 360 compares selected securities using dated weight references and available market data. Its charts and numbers are display and analysis tools; they do not execute trades or guarantee that a selected basket matches the official index.