A work sampling study is to be performed on the art department in a publishing company. The department consists of 22 artists who work at computer graphics workstations developing line drawings based on authors' rough sketches. The duration of the study is 15 days, seven hours per day. Line drawings are the main activity performed by the artists, but not the only activity. 500 initial observations were taken in which 375 showed artists making line drawings. At the 95% confidence level, how many more observations are required if management wants to insure that the estimate is within plus or minus 3% of the true value