Weighted Points and Effective Contribution
Learn how approximate constituent returns can be translated into percentage-point and index-point contributions.
Read the draftHow Market Watch 360 Calculates
How the Weighted Index Chart Works
Read the direction, reference line, and selected-stock scope of a weighted market chart without mistaking an estimate for the official index.
Draft updated · owner review pending
A weighted chart shows how selected securities combine under a chosen calculation over time. It can reveal when the leaders change from helping to hurting the market view. Its line is a selected-basket estimate unless the chart explicitly uses the complete official index series.
Identify the reference first
The combined chart is titled S&P 500 weighted move and shows estimated S&P points for the selected stock basket, measured against each stock's previous close. Read its All / Top 10 / Top 15 / Top 20 / Top 30 choice and 5 min / 15 min / 30 min / 1 hour interval before interpreting a peak. The time axis follows the PC's local time zone.
A simple two-stock path
Hypothetical example: Stock A begins at a 6% weight and rises 2%, contributing about +0.12 percentage points. Stock B begins at 3% and falls 1%, contributing about −0.03. Together they add an estimated +0.09 percentage points at that moment. If A later returns to flat while B remains down, the same selected basket becomes negative. The visual turn can happen without either stock changing its index membership.
Why the line and official close can diverge
A selected basket omits other constituents. Weight references may be ETF-based and dated; available stock trades or quotes can arrive at different times. The official index provider may use different data and corporate-action handling. A chart drawn from these inputs is useful for seeing the selected drivers, not a substitute for a licensed complete index feed.
Key takeaway
Treat the weighted chart as a time-based explanation of the followed S&P stocks, not an official S&P 500 history. Reset restores the full session and automatic price scale; Expand / Restore changes chart size, not the calculation.
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
- Alpaca Real-time Stock DataStock stream messages, feed types, and delayed SIP