A so-called c-chart or defects-per-unit chart is used for the control of the number X of defects per unit (for instance, the number of defects per 100 meters of paper, the number of missing rivets in an airplane wing, etc.).

(a) Set up formulas for CL and LCL, UCL corresponding to μ ± 3σ, assuming that X has a Poisson distribution.

(b) Compute CL, LCL, and UCL in a control process of the number of imperfections in sheet glass; assume that this number is 3.6 per sheet on the average when the process is in control.