Hernandez Cast Parts wants to compare the performance of four machine operators in the manufacturing section of the company, using a sample of 200 units produced by each operator. The number of defective units was as follows: Smith sample, 12; Roberts sample, 16; Buckley sample, 18; and Phillips sample, 10. Use the chi square test of proportion differences to test the hypothesis that the proportion of defective units for the population of units produced by each of the operators would be the same. Use a significance level of 1%.