By John S. McClenahen On April 15, economists were quick to characterize last week's unexpected rise in initial claims for unemployment insurance as a statistical anomaly in an otherwise improving U.S. labor market. For example, David A. Rosenberg, chief North American economist at Merrill Lynch & Co., New York, suggested that the Easter and Passover holidays along with spring break may have affected the way raw data were adjusted for seasonal variation. "Additionally, the first week of the [calendar] quarter also brings other volatility issues," he added. "Given these caveats, the latest figures should be viewed cautiously." Nevertheless, numbers from the Labor Department's Employment and Training Administration show the figure for initial claims was 360,000 for the week ending April 10, an increase from the previous week's revised figure of 330,000. The department's four-week moving average of initial claims also rose last week, rising to 344,250, an increase of 6,750 from the previous week's revised average of 337,500.