What it means
Headline CPI = all items in the consumer basket. Core CPI = headline EXCLUDING food and energy components, which are volatile (commodity-driven) and don't reflect underlying inflation dynamics. The Fed focuses on core because monetary policy works through underlying inflation, not commodity prices. Headline gets the press but core gets the policy reaction.
Why it matters
Markets initially react to headline (it's the first number on the wire) then re-adjust based on core. The two can diverge significantly in some months — high headline driven by oil prices while core is moderate, or vice versa. Knowing which is more informative for the Fed's reaction function helps anticipate the post-initial-headline market re-adjustment.
How to use it
On CPI release, focus on core (the 'YoY change in CPI excluding food and energy'). Compare core to consensus. Core surprise direction drives the Fed's policy reaction more than headline surprise. Watch for divergence: hot headline + cool core often produces an initial selloff that reverses; cool headline + hot core often produces an initial rally that reverses.
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