The milling machine in the fabrication department of your company produces a very important part for the products you sell. You are given the following historical data on the machine (from a SAMPLE of n = 25 weeks): Machine Breakdowns/week Number of parts (X) average # of machine breakdowns/week: X bar = 50 standard deviation of # of machine breakdowns/week: Sx = 15 n = 25 Produced/week (Y) average # of parts produced/week: Y bar = 170 standard deviation of # of parts produced/week: Sy=40 n = 25 What is the symbol we used to represent the population standard deviation for number of parts, produced by this machine in a week?