Market breadth

Breadth is built by sorting every stock in the tracked universe into buckets based on today's percentage move, big losers, moderate losers, roughly flat, moderate gainers, big gainers, and counting how many names fall into each. The result is typically shown as a histogram rather than a single number, since the shape of that distribution carries more information than any one summary statistic could. A market where most names cluster in the flat-to-moderate buckets looks very different from one where the distribution is bimodal, with clusters at both extremes and few names in the middle.

Read breadth alongside the headline index, not instead of it. A rising index with strong, broad-based breadth, most stocks green, the distribution skewed toward gainers, reflects genuine, widespread participation. A rising index with weak or negative breadth, meaning the gains are being carried by a small number of large-weighted names while the broader universe of stocks is actually declining, is a classic divergence that some traders treat as a caution flag, since narrow leadership has historically preceded periods of instability more often than broad-based advances have.

Breadth doesn't identify which specific stocks are driving a divergence, or why. A weak-breadth session could reflect genuine broad selling pressure, or it could simply reflect that day's sector rotation, money moving into a handful of large names while smaller-cap names lag for unrelated reasons. It's also a same-day snapshot. A single session of divergent breadth doesn't establish a trend on its own, and breadth readings can flip from broad participation to narrow leadership and back within the same week without signaling anything structurally different about the market.

Consider a session where the S&P 500 closes up 0.3%, but the breadth histogram shows 340 decliners against only 160 advancers, with the biggest gains concentrated among a small handful of mega-cap technology names. That's a divergence: the index number alone would suggest a modestly positive, uneventful day, while the breadth data shows the majority of individual stocks actually lost ground. Compare that to a session where the index is also up 0.3% but breadth shows 400 advancers against 100 decliners, a broad, participatory rally the same headline number doesn't distinguish from the first case.