Equal-weight vs cap-weight, and breadth thrusts

Two topics close the daily-breadth module. The first is the cleanest single chart for leadership: the ratio of an equal-weight index to its cap-weighted twin. The second is the most-hyped breadth signal there is, the breadth thrust, and what its record actually looks like when you count honestly.

The equal-weight ratio

An equal-weight index gives every member the same weight, rebalanced periodically. For the S&P 500 that is 0.2% each. Divide the equal-weight index (or its ETF) by the cap-weighted one and plot the ratio.

Ratio rising  ──►  average stock beating the megacaps: broad rally or megacap selloff
Ratio falling ──►  megacaps beating the average stock: narrowing rally or broad selloff
Ratio flat    ──►  leadership balanced

Note that a rising ratio is not automatically bullish: the average stock can "outperform" by falling less. Read the ratio alongside the index's own direction:

The second row is the quintessential late-cycle picture and the one this course has been circling since Module 1. It can persist for many months. The third row often appears at the start of a rotation, when leadership changes hands.

The ratio also has a long-cycle character: multi-year periods where the average stock leads (2000-2007 broadly) and multi-year periods where the megacaps lead (much of 2015-2024). For a swing trader that long cycle is background; the useful reads are the multi-week divergences against the index.

Key idea: The equal-weight to cap-weight ratio is the simplest picture of who is leading. Index up with ratio down is a narrow rally; index up with ratio up is a broad one. Trade the average stock more aggressively in the second case.

Breadth thrusts: the idea

A breadth thrust is a sudden, unusually broad surge in participation, typically off a low. The logic is that only a change in regime, not a bounce in a bear market, produces almost every stock rising at once for days on end. Several definitions exist:

They all measure the same thing: a violent switch from most stocks falling to almost all stocks rising.

The honest record

The Zweig thrust is the famous one, and its marketing line is that it had a perfect record, with the index higher six and twelve months later after every signal from the 1940s onward. That was true for a long time, on a sample of roughly a dozen signals in sixty years. Since around 2009 signals have become far more frequent, partly because ETF and program trading produce more days where "everything goes up together", and the record since then is good but not perfect.

Two honest caveats. First, "higher a year later" is a low bar in an index that rises most years anyway; the relevant comparison is against the base rate, and the thrust's edge over it is real but smaller than the perfect-record framing implies. Second, "higher in twelve months" says nothing about the path. A thrust can be followed by a 10% pullback and still count as a success, which is no use to a swing trader stopped out in week three.

What thrusts are genuinely good at: identifying that a regime has changed from bear to bull. The typical failure of a bear-market rally is that it is narrow; a thrust is by definition not narrow. When you see one after a long decline, the correct update is "the regime call moves from downtrend to probable uptrend" and the correct trade is to stop shorting rallies and start buying pullbacks, with normal stops. It is not "go all in".

Thrusts sketched

Index:      ╲╲╲╲___╱╱╱╱╱‾‾╱╱╱
% up 10d:   .35 .33 .38 .44 .52 .58 .63 .65   ← crosses 0.615 within 10 days: thrust
% > 50-day: 14% 12% 18% 31% 47% 62% 74% 83%   ← surge from washout to stretched
90% days:                ▲       ▲             ← two 90% up-volume days

All three definitions fire within the same fortnight. That clustering is the useful pattern: when several independent thrust measures agree, the signal is stronger than any one of them.

Thrust failure modes

• Counting near-misses. A 10-day ratio that reaches 0.60 is not a Zweig thrust. Traders who relax the rule after the fact inflate the record.
• Thrusts without a prior washout. Most definitions require starting from a low (below 0.40, below 20%). A surge from neutral is ordinary strength, not a thrust.
• Ignoring the path. Size positions on the assumption that a normal pullback will follow, because it usually does.
• Expecting them often. Real thrusts are rare. If you are seeing one every few months, your definition is too loose.

Try it: Compute the 10-day EMA of advancers ÷ (advancers + decliners) for the last two years of NYSE data (the smoothing factor is 2 ÷ 11 ≈ 0.18). Mark every time it crossed from below 0.40 to above 0.615 within 10 sessions. For each mark, record the index's maximum drawdown over the next 60 sessions as well as its return. The drawdown column is the one most write-ups leave out.

Recap

• The equal-weight to cap-weight ratio shows who is leading; read it with the index's direction, since a rising ratio can mean falling less.
• Index up with ratio down is a narrow rally, the late-cycle picture; index up with ratio up is broad and healthy.
• Breadth thrusts (Zweig, Deemer, Whaley, 90% days, % above 50-day surges) measure a violent switch to near-universal participation off a low.
• Their forward record is good but the perfect-record framing overstates it: samples are small, signals are more frequent since 2009, and the path can include large pullbacks.
• Use a thrust to flip the regime call from bear to bull and to change tactics, not as a reason to abandon sizing rules.