Using an injection molding process, a plastics company produces interchangeable cell phone covers. After molding, the covers are sent through an intricate painting process. Quality control engineers inspect the covers and record the paint blemishes. The number of blemishes found in 20 samples of 5 covers are as follows: 2, 1, 5, 5, 3, 3, 1, 3, 4, 5, 4, 4, 1, 5, 2, 2, 3, 1, 4, 4.
(a) Using all the data, compute trial control limits for a U control chart, construct the chart, and plot the data.
(b) Can we conclude that the process is in control using a U chart? If not, assume that assignable causes can be found, list points, and revise the control limits.