What Is Weighted Index Contribution?
A simplified estimate multiplies a security's index weight by its percentage return.
Read the guideHow Market Watch 360 Calculates
Weighted Points and Effective Contribution
Translate a simplified weighted return into index-point context and learn why an application estimate can differ from the published index.
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Weighted contribution answers how much a followed stock's move contributes to the selected S&P basket estimate. Converting percentage-point contribution into S&P points requires a reference index level; the desktop app labels the result as an estimate unless a reviewed reference set is available.
Keep the three units separate
A stock return is the security's percentage change. A weighted percentage-point contribution is starting weight multiplied by that return. An index-point estimate multiplies the contribution expressed as a decimal by a reference index level. Each step changes the unit, so a single number cannot stand for all three.
Work through a hypothetical case
Suppose a stock begins with a 5% weight and rises 2%. Its simplified contribution is 0.05 × 0.02 = 0.001 of index return, or +0.10 percentage points. At a hypothetical starting index level of 5,000, that is about 5 index points (5,000 × 0.001). Other securities can add or subtract more, and the official index still follows its full methodology.
Why the effective value can differ
The desktop app measures each followed stock against its previous close, multiplies by its IVV-based weight, and converts the total to S&P points. Without a reviewed reference set, its point scale uses SPY's prior daily close × 10, rather than SPY's live price. A reviewed index level with near-synchronous stock prices can replace that fallback. The basket omits other constituents and does not reproduce the official index provider's calculation.
Key takeaway
Read 'effective contribution' as a labeled estimate with a declared reference level and selected universe. Do not substitute it for the official index's point change.
Sources and further reading
- S&P DJI Index Mathematics MethodologyOfficial index calculation and divisor reference
- Nasdaq-100 Index MethodologyOfficial modified market-cap weighting and constituent rules