High-Low Logic Index

The High-Low Logic Index, developed by technician Norman Fosback, is a ratio that divides new 52-week lows by the sum of new highs plus new lows, creating a single percentage that flags when market extremes are becoming skewed—a contrary warning that strength may be deteriorating even as indices rally.

Why the ratio tells a story

In a straightforward bull market, new 52-week highs overwhelm new lows. If 150 stocks hit highs and only 20 hit lows on a given day, the ratio is 20 ÷ (150 + 20) = 11%. The market has clear breadth winners.

But when a rally is losing conviction, new highs and lows begin to balance out. If 80 stocks hit highs and 70 hit lows, the ratio jumps to 70 ÷ (80 + 70) = 46%—nearly half of all extremes are on the downside. The High-Low Logic Index captures this skew. When it climbs above 40% or 50%, technicians read it as an alarm: fewer stocks are participating in the advance. Trouble may be brewing.

Fosback’s contrary logic

Fosback observed that extreme readings of the High-Low Logic Index—when new lows dominate the extremes—often come at or near market bottoms. When fear peaks and the index soars to 60%, 70%, or higher, capitulation is underway. These are often the best times to buy, not sell. Conversely, very low readings (below 10–15%) can signal complacency or an overheating rally that has exhausted breadth.

The index is most powerful as a contrary indicator: the most uncomfortable reading is often the most actionable.

Divergences with price

The real edge comes when the High-Low Logic Index diverges sharply from price action. The S&P 500 is making new all-time highs, yet the index shows 45% new lows. That’s a warning: fewer stocks are joining the party. Leadership has narrowed to a small subset. History suggests these rallies often stall or reverse within weeks.

Likewise, when the index spikes above 50% but large-cap indices hold above key support levels, it’s a sign that selling is concentrated in smaller, less liquid, or more vulnerable stocks. The correction may be localized and tradeable.

Using it with other breadth signals

The High-Low Logic Index works best alongside the advance/decline line, NYSE TICK, and Hindenburg Omen. A day with an extreme High-Low Logic reading (70%+ new lows) plus a negative advance/decline plus a second Hindenburg Omen in a week is a serious convergence of bearish breadth—far more convincing than any single measure alone.

Technicians often plot a moving average of the index (10-day or 20-day) to smooth daily noise and spot true trend changes in breadth extremes.

Practical thresholds

While Fosback’s original research cited specific levels, practitioners have refined the framework:

• Below 15%: Bullish extremes; few new lows; rally may be overextended but is broad-based.
• 15–35%: Healthy, normal; no immediate warning.
• 35–50%: Yellow flag; breadth deterioration; watch other indicators.
• 50%+: Red flag; likely bottoming conditions or significant correction underway; contrarian buy signal at extremes.

These are guides, not absolutes. Context matters: a 50% reading after a 5% gain feels different from a 50% reading after a 15% gain from lows.

When the signal misfires

The High-Low Logic Index, like all breadth measures, can meander in choppy, directionless markets. It may stay elevated (40–50%) for weeks without resolving into either a crash or a new high. It works better in trending markets where participants are more aligned and reversals are cleaner.

Also, in periods of heavy sector rotation (e.g., a tech-to-value shift), the index can be skewed by concentrated selling in one area, even if the broader market is rebalancing healthily. Good trading requires reading the breadth and understanding what’s driving it.

Integration into a process

Professional technicians treat the High-Low Logic Index as one dial on a dashboard. It doesn’t stand alone. It confirms or questions what the price-to-earnings ratio, yield curve, credit spreads, and sector leadership are telling you. When all arrows point the same direction—breadth weak, valuations stretched, spreads widening—the case for caution is strong. When breadth signals conflict with price momentum, it’s time to dig deeper.

See also