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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 & 11Market breadth measures how many or what proportion of securities are participating in a market move. It can reveal whether a rising headline index reflects broad gains or is being carried by a smaller group of large constituents. Breadth is a way to assess participation—not a standalone forecast of what prices will do next.
What market breadth measures
Breadth is an internal-market measure: it counts securities moving in a specified direction or meeting a defined condition within a named universe, such as S&P 500 constituents, Nasdaq-listed issues, or stocks in a sector.
A capitalization-weighted index reflects the combined value changes of its constituents, so a few large companies can lift the index even when many other stocks lag. Breadth adds a different view by showing how widely the move is shared. For example, Nasdaq’s October 13, 2021 article compared the S&P 500 with an advance/decline line based on S&P 1500 stocks, illustrating that the breadth universe and the headline index do not have to be the same. Those measures should be named separately when compared. Nasdaq’s discussion of market breadth also reported that 41% of S&P 500 stocks were above their own 50-day moving averages at the time, versus nearly 70% a little over a month earlier. These are historical readings from 2021, not current figures.
How to measure breadth
1. Count advancing and declining issues
For each security in the chosen universe, compare its current-session close with the previous session’s close. Count a higher close as an advance and a lower close as a decline. Report the counts with the universe, date, and convention for unchanged issues. TradingView’s documented convention groups unchanged issues with decliners; other data providers may classify them differently. TradingView’s advance/decline indicator documentation explains its definitions.
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Basic counts give every security one vote, regardless of company size or how much its price changed. They are useful for participation, but do not show the size of moves or the constituents’ index weights.
2. Calculate net advances and the A/D ratio
- Net advances = advancing issues − declining issues.
- Advance/decline (A/D) ratio = advancing issues ÷ declining issues.
A positive net-advance figure means advances outnumber declines under the stated classification. An A/D ratio above 1 means more issues advanced than declined; below 1 means more declined. If there are no declining issues, the ratio is undefined, so report the counts rather than divide by zero.
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3. Build a cumulative A/D line
The A/D line accumulates daily net advances:
A/D line today = A/D line yesterday + (advancing issues today − declining issues today)
The first value is a chosen baseline, so the line’s absolute level depends on where the series starts. Readers generally focus on its slope, trend, highs and lows, and relationship to a named index—not on comparing the absolute values of differently initialized series or data sources. Fidelity’s guide to the A/D line describes its use in confirming trends or spotting possible divergences, while cautioning that it does not always confirm a trend or forecast a reversal.
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4. Measure the percentage above a moving average
This measure counts the share of a universe whose constituents’ latest closes are above their own moving averages:
Percentage above MA = (constituents with a latest close above their own MA ÷ total constituents) × 100
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The moving-average period defines the horizon. TradingView documents 20-day, 50-day, and 200-day simple moving-average versions, which it characterizes as short-, medium-, and long-term, respectively. State both the universe and lookback: a statement such as “breadth is 60%” is incomplete without them. TradingView’s moving-average breadth documentation explains the measure.
5. Add volume or new-high/new-low measures
Volume breadth weights by traded volume rather than counting each security equally. TradingView defines up volume as the volume of advancing constituents divided by total constituent volume, and down volume similarly for declining constituents. These measures answer a different question from A/D counts: how much volume is associated with each side, rather than how many securities moved that way. TradingView’s indicator definitions cover these calculations.
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New 52-week highs and lows can provide additional context. The Boston Federal Reserve’s historical glossary describes a 10-day average of issues on an index or exchange making new 52-week highs or lows; it explicitly notes that the described relationships reflect analysts’ thinking and were not necessarily endorsed or validated by the Reserve Bank. Treat that page as a historical glossary, not as validation of a trading signal. Boston Fed glossary and endnotes
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to interpret breadth alongside an index
- Index rising, A/D line rising: More securities are contributing to the advance in the selected universe, consistent with broad or broadening participation.
- Index rising, A/D line flat or falling: Participation may be narrowing. Check whether a few large constituents or particular sectors are carrying the index; this pattern alone is not a sell signal.
- Index falling, A/D line rising: Fewer securities may be declining than before. That can be consistent with waning selling pressure, but it does not establish that a bottom is in.
- Index and A/D line diverging: For example, an index may make a higher high while its A/D line makes a lower high. Fidelity describes this as a possible sign of a weakening rally, but says it may not predict a reversal.
- Many constituents above a longer moving average: A larger share of the chosen universe is above that trend threshold. The reading is not automatically a buy signal; extreme readings can also reflect a stretched market, while low readings can accompany widespread selling.
Nasdaq authors Brandon Bischof and Tom Hardin wrote that “The A/D Line often indicates how ‘healthy’ the market is at a given point.” That is their description of how the indicator is used, not an objective or predictive measure of market health. Their October 13, 2021 article also reported that three sectors—Information Technology, Communications Services, and Health Care—accounted for 52% of S&P 500 market capitalization in the period discussed. That figure is specific to their historical example.
Make comparisons meaningful
Before comparing two breadth readings, check that they use compatible definitions:
- Universe: Exchange-wide issues, an index, a sector, or another constituent group. A market-wide A/D line and an index’s price are not a like-for-like comparison unless both universes are identified.
- Horizon: A daily change, a cumulative A/D trend, and the share above a 20-, 50-, or 200-day average describe different time frames.
- Weighting: Basic A/D counts give each security one vote; volume breadth weights by traded volume.
- Classification: Check how the provider handles unchanged issues and any other calculation rules.
- Data continuity: Use a consistent source and universe over time, and account for changes in index membership or data coverage.
Breadth describes participation; it does not explain why securities moved or guarantee that a trend will continue or reverse. Fidelity cautions that A/D signals may fail to confirm trends or forecast reversals. The Boston Fed’s glossary likewise disclaims endorsement or validation of the technical-analysis relationships it describes. The sources cited here do not establish a general performance statistic showing that breadth predicts future market returns.
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