The S&P 500 and the Nasdaq Composite are suffering the biggest one-day selloff in three months, but market breadth data suggests the sellers are acting relatively calm and collected. The Arms Index, which is a volume weighted breadth measure, inched up to just 1.103 for the NYSE and edged up to 1.146 on the Nasdaq exchange. An Arms of 1.000 means the ratio of advancers over decliners is is the same as the ratio of advancing volume over declining volume; basically, there's no sense of urgency to sell stocks that are falling. A rise in the Arms to 2.000 suggests panic-like selling. The S&P 500 was down 1.3% and the Nasdaq Composite 2.0%, the biggest declines for both since May 17. The last time the NYSE Arms was above 2.00 was April 13, when the S&P 500 slumped 0.7%, while the last time the Nasdaq Arms was above 2.000 was June 9 when the Nasdaq Comp fell 1.8%.
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