An index can rise because a few heavyweight stocks rise. Breadth tells you whether most stocks are joining in.
- Advance / decline: stocks up versus stocks down today. A ratio above 1.5 is a broad rally; below 0.7 a broad fall.
- % of stocks above the 200-day average: above 60–70% is a healthy, broad uptrend; below 30% a weak market.
- An index making new highs while breadth weakens is a common early warning.