Inflation has remained stubbornly low even as slack in the labor market appears to have been much diminished. Core inflation as measured by the PCE price index, which strips out volatile food and oil prices, rose to a 1.3% annual rate in the third quarter, up from 0.9% in the second quarter. More from Yellen here:

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Yellen says inflation below 2% goal poses one of Fed’s ‘biggest challenges’ - MarketWatch
As Federal Reserve Chairwoman Janet Yellen’s tenure nears its end, she said one of the biggest challenges to the central bank remained inflation staying below 2%