貨幣政策 失業率 - 經濟

各位版友好，
課本寫說:
If the unemployment rate is below the natural unemployment rate, a labor shortage might put pressure on wage rates to rise, which might feed into inflation.The Fed might consider raising the federal funds rate.
If the unemployment rate is above the natural unemployment rate, a lower inflation rate is expected. The Fed might consider lowering the federal funds rate.
可是失業率不是會大於等於自然失業率嗎?
怎麼會有可能unemployment rate is below the natural unemployment rate

感謝解惑!

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