We have taken 12 samples of 400 letters each from a typing pool and found the following proportions of defective letters: .01, .02, .02, .00, .01, .03, .02, .01, .00, .04, .03, and .02. A letter is considered defective when one or more errors are detected.
a. Calculate the control limits for a p control chart.
b. A sample of 400 letters has just been taken, and 6 letters were found to be defective. Is the process still in control?