Market Breadth

What Is Market Breadth?

Market breadth measures how many stocks are participating in a market move, whether a rally or decline is broad-based (most stocks moving together) or narrow (driven by a handful of large-cap names while most stocks diverge). It is one of the most important tools for assessing the health and sustainability of a market trend.

A market index like the S&P 500 is capitalization-weighted, the largest companies (Apple, Microsoft, Nvidia, Amazon) have enormously disproportionate influence. The S&P 500 can rise 20% while the median stock gains only 5%, or even declines, if the mega-caps rally enough to mask broad weakness. Breadth analysis penetrates this distortion to reveal what the "average stock" is actually doing.

Core Breadth Indicators

1. NYSE Advance-Decline (A/D) Line

The most widely followed breadth measure. Calculated daily as:

A/D Line = Previous A/D + (Advancing Stocks - Declining Stocks)

Historical divergence signals:

• 1999-2000: A/D line peaked in April 1998, 23 months before the S&P 500 topped in March 2000. The dot-com rally's final phase was driven entirely by large-cap tech while the average stock was already in a bear market.
• 2007: A/D line peaked in June 2007, 4 months before the S&P 500's October 2007 all-time high.
• 2021: A/D line peaked in November 2021, coinciding with the S&P 500 peak. No divergence (the sell-off was broad-based from the start).

2. Percentage of Stocks Above Key Moving Averages

Extreme readings:

• March 2020 COVID crash: only 2% of S&P 500 stocks above their 200-day MA, the deepest oversold reading since 2008
• January 2021 post-stimulus rally: 95% above 200-day MA, one of the broadest participation rates in decades
• Late 2023: S&P 500 at all-time highs but only 55% above 200-day MA, narrow participation

3. Cap-Weighted vs Equal-Weighted S&P 500

The simplest breadth check: compare SPY (cap-weighted) to RSP (equal-weight). When they perform similarly, breadth is healthy. When SPY significantly outperforms RSP, mega-caps are doing the heavy lifting.

The 2023 spread of 14 points was the largest since the dot-com bubble, a historically extreme concentration signal.

4. New 52-Week Highs Minus Lows

The daily count of NYSE stocks making new 52-week highs minus those making new lows. In a healthy bull market, net new highs should be expanding. A shrinking number of new highs during an index rally is a divergence warning.

5. McClellan Oscillator and Summation Index

Advanced breadth momentum indicators:

• McClellan Oscillator: The difference between the 19-day and 39-day EMAs of the daily advance-decline data. Readings below -100 = oversold (bounce expected); above +100 = overbought.
• McClellan Summation Index: The cumulative sum of the McClellan Oscillator. Rising = breadth momentum improving. Falling = breadth deteriorating.

The Magnificent Seven Concentration Problem

The 2023-2024 market presented the most extreme concentration since the Nifty Fifty era of the early 1970s:

The Nifty Fifty parallel: In 1972-1973, a group of ~50 "one-decision" stocks (IBM, Xerox, Polaroid, Coca-Cola) traded at 40-80x earnings and dominated the market. When the 1973-1974 bear market hit, these stocks fell 50-90%, far more than the broader market. The concentration that made them appear safe was actually a fragility, when the narrative broke, everyone was selling the same stocks.

Does the Magnificent Seven end the same way? Not necessarily, unlike the Nifty Fifty, the Mag 7 are genuinely the most profitable companies in history (Apple, Microsoft, Alphabet, and Meta collectively generate $300B+ in annual free cash flow). The risk is not that they are bad companies but that their weight in the index means any correction in this small group creates outsized index damage.

Breadth Thrusts: The Most Bullish Signal in Technical Analysis

A breadth thrust occurs when market participation surges from deeply oversold to strongly positive within a compressed timeframe. The most famous formulation:

Zweig Breadth Thrust: 10-day MA of NYSE advances/(advances + declines) rises from below 0.40 to above 0.615 within 10 trading days.

Win rate: 100% positive 6-month returns across all 15 occurrences since 1945. Average 12-month return: +24%.

What to Watch

1. NYSE A/D line vs S&P 500, if the S&P makes a new high and the A/D line doesn't, reduce risk
2. SPY vs RSP spread, a gap exceeding 10 points over trailing 12 months signals extreme concentration risk
3. % above 200-day MA, below 20% = oversold buy zone; above 80% = overbought caution zone
4. Breadth thrust signals, rare but extremely bullish; when triggered, increase equity exposure and hold for 6-12 months
5. New high/new low ratio, expanding new highs = stay bullish; expanding new lows during a rally = danger