By John S. McClenahen For the unemployed, the statistics are of little consolation. But U.S. Labor Department data show the number of mass layoffs last month declined to 920 from 941 in February, the best showing for any March since 1999. With 258 mass layoffs in March, U.S. manufacturing accounted for 28% of the total. The largest number -- 56 -- was in food and the next highest number -- 38 -- was in transportation equipment. By the department's definition, each mass layoff involves at least 50 people from a single establishment.