A hospital laboratory has the task of screening blood samples of patients and producing a summary report for clinicians. Because of the limitations of equipment, staff training and time, the laboratory manager knows that there is an error rate involved in the production of such reports. It is estimated that 4 per cent of such reports produced are incorrect in terms of their results. Over the past few months the lab has been under increased pressure from clinicians to reduce the turnaround time in the production of the test reports. The manager is concerned that this improvement has been at the expense of an increase in the error rate. Accordingly, Error rate Percentage Day 1 6.9 Day 2 4.3 Day 3 6.6 Day 4 5.2 Day 5 6.8 Day 6 8.5 Day 7 8.9 Day 8 9.5 Day 9 9.8 Day 10 11.5 using the principles of statistical process control, it has been decided on an experimental basis to take samples of 50 reports a day and double-check them to monitor the error rate. The results for the first two weeks are as follows:

(a) Construct a suitable control chart for use by the lab.

(b) Do you think that control charts would be useful and reliable in this context?