Market Breadth
Also called: advance decline line, breadth indicators, percent above 200-day
Market breadth measures how many stocks participate in a move, through advance-decline lines or the share trading above a moving average. Narrow breadth — an index rising on a handful of mega-caps while most stocks fall — is a classic late-cycle warning. It matters more now than historically, because cap-weighted indexes have become concentrated enough to hide a broad decline entirely.