Internal Strength Index

An internal strength index (ISI) measures the ratio of advancing stocks to declining stocks in a market or sector, indicating whether the broad base of equity participants are supporting a price trend or refusing to participate.

What breadth measures that price does not

The S&P 500 can rally while only 30% of stocks participate—the heaviest-weighted constituents drag the index higher even as the median stock declines. This is breadth divergence, a critical sign of weakening trend. The internal strength index captures this divergence by explicitly counting stock-level participation.

An index with 400 advancing stocks and 100 declining stocks has an ISI of 4.0 (400:100). An index with 250 advancing and 250 declining has an ISI of 1.0 (even participation). An index with 150 advancing and 350 declining has an ISI of 0.43 (declining stocks dominating).

Most technicians interpret ISI readings as:

• ISI > 2.0: Very strong breadth, trend is broad-based.
• ISI 1.0–2.0: Neutral to moderately strong breadth.
• ISI 0.5–1.0: Weak breadth; index may be driven by large-cap strength alone.
• ISI < 0.5: Very weak breadth, often a warning of reversal.

Breadth divergence as a warning signal

A classic pattern: the S&P 500 rallies to all-time highs while ISI falls from 1.8 to 0.9. This breadth divergence—price making new highs while fewer stocks advance—is historically a precursor to correction. The rally lacks conviction; it is driven by a narrowing set of mega-cap names.

This pattern dominated 2023–2024, when “Magnificent Seven” mega-cap tech stocks drove the S&P 500 higher while the median NASDAQ stock lagged, and most small-cap and value sectors underperformed. The index breadth (ISI) compressed, signaling that the rally was not healthy and sustainable.

Breadth thrust and reversal patterns

A breadth thrust occurs when ISI spikes sharply from weak readings (0.4–0.6) to strong readings (2.0+) in a single day or over a few days. This often marks the beginning of a strong, broad-based rally and is considered a bullish signal by many technicians. A breadth thrust that occurs after a capitulation selloff (when fear is extreme) is especially potent.

Conversely, a breadth breakdown occurs when ISI collapses from strong readings (>1.5) to weak readings (<0.8) in a short period. This often accompanies the start of a correction and is considered a sell signal.

Daily and cumulative breadth metrics

Traders often plot the cumulative advance-decline line (AD line), which is a running sum of (advances − declines) each day. Unlike ISI, which is a ratio, the AD line is a cumulative measure. Both are used in breadth analysis, but they answer slightly different questions:

• ISI (ratio): What percentage of stocks are advancing today?
• AD line (cumulative): Has the trend of advancing stocks been persistent over weeks/months?

An ISI of 1.2 today is moderately bullish; but if the AD line is making new lows, the market has been weak for weeks despite today’s slight breadth advantage. Combining both metrics is more powerful than using either alone.

Breadth in sectors and industries

Investors can calculate ISI for subsets: the financials sector, technology sector, or a specific industry group. A sector-level ISI reveals internal strength or weakness. During a sector rotation, the retreating sector’s ISI will often roll over weeks before price deteriorates sharply. This leading-indicator property makes sector ISI valuable for tactical allocation.

Example: During the 2022 tech selloff, tech-sector ISI collapsed from 0.8 (modest breadth support) to 0.3 (severe breadth collapse) in November 2022, presaging the final sharp leg down in late 2022 and early 2023.

NYSE/NASDAQ breadth data sources

The New York Stock Exchange (NYSE) publishes daily breadth data: advances, declines, and unchanged issues. The NASDAQ does the same. These are publicly available and widely tracked:

• NYSE advancing: ~1,500–2,000 stocks typically advance on a strong day
• NYSE declining: ~1,500–2,000 stocks typically decline on a weak day
• NASDAQ: Similar scale, but NASDAQ includes many more speculative, small-cap names, making its breadth more volatile

During strong bull markets, daily NYSE advances can exceed 2,500 (out of 3,500 total), generating an ISI > 1.5. During bear markets, advances can fall below 1,000, generating an ISI < 0.4.

Limitations and noise

ISI is noisy day-to-day and benefits from smoothing. A 3-day or 5-day moving average of ISI reduces intraday whipsaws. Weekly ISI (based on the week’s advances vs. declines) is more stable and useful for intermediate-term trend assessment.

Additionally, ISI is biased by the composition of the exchange. If the NASDAQ is heavily weighted to mega-cap tech, and those stocks are rallying, NASDAQ ISI will look strong even if 90% of mid-cap and small-cap stocks are declining. Breadth at the sub-index level (large-cap NASDAQ vs. small-cap NASDAQ) is more informative.

Using ISI for tactical exits and reversals

Many technicians use ISI readings to time exits or position reversals. A strategy: when the S&P 500 is near all-time highs but NYSE ISI falls below 1.0, exit long positions or establish a small short bias, expecting a correction. If ISI subsequently recovers to >1.5, re-establish the long bias.

This breadth-based tactical trading has some historical edge, but performance depends on regime. In very strong bull markets, ISI can stay weak for weeks while prices continue rising (because mega-cap stocks are dominant). In weak bear markets, ISI can briefly spike without reversing the downtrend.