Market Breadth

Market breadth measures how many stocks are participating in a market move—basically, whether gains or losses are spread across most companies or concentrated in just a few. You'll hear analysts mention it when they're trying to figure out if a rally is healthy and sustainable. It matters because if the overall market is up but only a handful of mega-cap stocks are driving it, that's a warning sign. Think of it like this: if the S&P 500 index rises 2%, but breadth shows that only 30% of stocks in it actually went up while the rest fell, that suggests the rally is fragile. Strong breadth—where most stocks move together—usually signals a more durable trend.