By John S. McClenahen There's still another sign that the robust rate of U.S. economic growth in the first calendar quarter of the year is not being maintained. The economy's manufacturing sector continued to grow during April, its third consecutive month of growth, but not as strongly as in March, reveal new data from the Institute for Supply Management (ISM), Tempe. Ariz. The group's closely watched PMI index was 53.9% in April, down 1.7 percentage points from 55.6% in March. It also was a full percentage point below the 54.9% that economists had generally expected. A figure above 50% indicates manufacturing is expanding; below 50% signals contraction. The new orders index, one of the PMI's 10 elements, lost momentum between March and April, falling to 59% from 65.3%. However, "It is still strong enough to drive sector growth in the next several months," claims Norbert J. Ore, group director of strategic sourcing and procurement at Georgia-Pacific Corp. and chairperson of ISM's manufacturing business survey committee. ISM's export and import indexes, two other components of the PMI, both grew in April -- although import growth outpaced export orders. The export-order index increased 0.9 percentage points to 51.9%. The import index rose 2.3 points to 55.7%.