Measures real output of US manufacturing, mining, and utilities. Index base: 2017=100. • Core indicator showing manufacturing activity through actual production data • Both month-over-month change and index level matter • Drops sharply in recessions — fell -17% during the 2008 financial crisis • Unlike ISM PMI (survey-based), this measures actual physical output Published directly by the Fed, it's the only monthly indicator that captures the production side of GDP.