Assignment task: Garcia's Garage desires to create some colorful charts and graphs to illustrate how reliably its mechanics "get under the hood and fix the problem." The historic average for the proportion of customers that return for the same repair within the 30-day warranty period is 0.10. Each month, Garcia tracks 100 customers to see whether they return for warranty repairs. The results are plotted as a proportion to report progress toward the goal. If the control limits are to be set at 2 standard deviations on either side of the goal, determine the control limits for this chart. In March, 8 of the 100 customers in the sample group returned for warranty repairs.
1. To determine if the process is in control, what type of chart (x-bar chart, R-chart, p-chart, or c-chart) should you be using?
2. Write down the UCL, CL, and LCL for a 3-sigma chart, use chart type you chose in question 1.
3. As stated in the book, 8 out of 100 customers returned for warranty repairs in March. Given this new information and the control chart in question 2, is the process in control? Explain why or why not.