In order to quantify the effects of part proliferation on assembly line operators' decision making capabilities, a research study was conducted. Using a Choice Reaction Time technique, 16 operators were tested to determine their reaction times and error rates when selecting parts. These operators were from four training levels (trained, relief, untrained/job and untrained/plant) and had to decide between 4, 7 or 10 major parts.Results show that operators with 10 parts made 46% more errors and needed 13% more decision time than operators with 4 parts. Furthermore, the relief and untrained/job operators made three times more errors than the trained operators. The untrained/plant operators had over five times more errors than the trained operators. These results indicate that all operators could make a selection when working with 10 major parts. However, their reaction times and error rates increased as the number or parts increased from 4 to 10.