New York--Jan. 4--A key barometer of the health of the U.S. non-manufacturing economy declined in December, indicating that the sector is still growing but continues to slow down. The National Association of Purchasing Management said its index of non-manufacturing economic activity fell to 53.0 in December from 58.5 in November. The monthly report, which is a counterpart to NAPM's widely watched monthly index of manufacturing activity, also said the price index was 58.0, up slightly from 57.0 in November.