"The idea that the full-employment unemployment rate (NAIRU) is not a unique number has been seen in recent empirical research. Staiger, Stock, and Watson found that the range of possible values of the NAIRU (from 4.3 to 7.3% unemployment) was too large to be useful to macroeconomic policy-makers. Robert Eisner suggested that for 1956-95 there was a zone from about 5% to about 10% unemployment between the low-unemployment realm of accelerating inflation and the high-unemployment realm of disinflation. In between, he found that inflation falls with falling unemployment."
Here's a BLS report that shows that more clearly: http://data.bls.gov/PDQ/servlet/SurveyOutputServlet?request_...