Market breadth

Market breadth measures how many stocks are taking part in a market move, rather than how far an index moved. Common measures are the share of stocks above their 50-day average, advancing against declining stocks, new highs against new lows, and counts of stocks up or down 4% in a day.

An index weighted by company size can rise on a few large stocks while most stocks fall. Traders use breadth to decide how hard to press: more and larger positions when most stocks are rising, fewer when most are falling.