INDICATOR EXPLAINER · #02
Market Breadth — is the average stock keeping up?
Breadth measures whether the whole market is participating in a move, or whether a handful of mega-caps are dragging the index around. 23 years of data reveal a surprising "smile" — extremes on both sides predict strong forward returns.
Today's reading
Same number you'll see on this component's card on the main dashboard.
What is market breadth?
The S&P 500 is a cap-weighted index — bigger companies count more. A small handful of mega-caps (Apple, Microsoft, Nvidia, etc.) can push the whole index higher even when most stocks are flat or falling. That's called a "narrow" rally, and it's historically fragile.
"Breadth" measures whether the average stock is participating. When breadth is broad, the rally is healthy. When breadth is narrow, the market is vulnerable — the mega-caps holding up the index can't do it forever.
BATS uses a fast, robust proxy: the RSP vs SPY spread.
The RSP vs SPY spread — how it works
- RSP Invesco S&P 500 Equal-Weight ETF — every stock gets equal weight, so it tracks the average stock's performance.
- SPY The classic S&P 500 ETF — cap-weighted, so it reflects the giants.
- Spread
20-day return of RSPminus20-day return of SPY. Positive = average stock is winning (broad). Negative = giants are winning (narrow).
How BATS uses breadth
Breadth is confirmatory, not contrarian: broad breadth confirms a healthy uptrend, narrow breadth warns that the rally may be running on fumes. The scoring is linear and centered on zero:
BATS points = 50 + (spread × 10), clamped to 5–95.
| Spread range | BATS points | Bucket |
|---|---|---|
| Above +4.5% | 95 | Extended |
| +2.5% to +4.5% | 75 – 95 | Bullish |
| +0.8% to +2.5% | 58 – 75 | Slightly Bullish |
| −0.8% to +0.8% | 42 – 58 | Neutral |
| −2.5% to −0.8% | 25 – 42 | Slightly Bearish |
| −4.5% to −2.5% | 5 – 25 | Oversold |
| Below −4.5% | 5 | Extremely Oversold |
Does it work? — Historical backtest (2003–2026)
For every trading day since RSP's April 2003 inception, we compute the breadth component of the BATS and measure how the S&P 500 actually performed 6 and 12 months later.
What the data actually says
Three findings jump out of the 23-year breadth backtest — and one of them is genuinely surprising:
The "smile" — both extremes are bullish.
Very Oversold breadth (narrow leadership at its extreme): +27.7% avg 12mo return, 99% positive. Extended breadth (broad participation at its extreme): +21.4% avg 12mo, 93% positive.
Takeaway: Extreme readings in either direction have historically preceded strong forward returns — for different reasons (mean reversion after narrow rallies; trend confirmation after broad ones).
The middle is baseline — nothing special.
When breadth sits near zero (average stock keeping pace with the index), forward returns track the baseline of roughly +10% at 12mo. Neither a strong signal nor a warning.
Takeaway: Breadth is most useful when it's at an extreme. Middle readings are noise — look to other indicators.
Complementary to the VIX.
The VIX only fires as a strong buy signal at one extreme (high VIX / panic). Breadth fires at both extremes. That means the two indicators light up in different market conditions and cover each other's blind spots.
Takeaway: Combining VIX with breadth captures more of the forward-return signal than either indicator alone.
How to use this information
Market breadth is famous for what it reveals about rally quality. Here's how to read the BATS breadth signal in a balanced way:
- Extreme readings matter most. The "smile" pattern is very real: extremes in either direction have historically preceded strong forward returns. Pay attention when the spread pushes past ±3%.
- Narrow rallies aren't automatically doomed. A negative spread means the giants are pulling the index; it doesn't mean the index is about to crash. Historically, extreme narrow rallies have often resolved with a broadening rally — mean reversion favors the average stock.
- Broad rallies confirm a trend. When the average stock is outrunning the mega-caps, the rally is healthy. Trend continuation is more likely from here.
- Pair with the VIX. Breadth and volatility measure different things and light up at different times. The BATS blends them so each can filter the other's noise.
- Middle readings are just noise. When the spread is near zero, breadth isn't telling you much. Look to other components (or wait for a clearer signal).
Reminder: None of this is investment advice. Historical patterns are not guarantees. Always do your own research or talk to a financial professional before investing.