Statistical process control techniq

Statistical process control techniques help managers achieve and maintain a process distribution that does not change in terms of its mean and variance. The control limits on the control charts signal when the mean or variability of the process changes. However, a process that is in statistical control may not be producing services or products according to their design specifications, because the control limits are based on the mean and variability of the sampling distribution, not the design specifications. Process capability refers to the ability of the process to meet the design specifications for a service or product.Design specifications often are expressed as a nominal value, or target, and a tolerance, or allowance above or below the nominal value. For example, the administrator of an intensive care unit lab might have a nominal value for the turnaround time of results to the attending physicians of 25 minutes and a tolerance of {5 minutes because of the need for speed under life-threatening conditions. The tolerance gives an upper specification of 30 minutes and a lower specification of 20 minutes. The lab process must be capable of providing the results of analyses within these specifications; otherwise, it will produce a certain proportion of “defects.” The administrator is also interested in detecting occurrences of turnaround times of less than 20 minutes because something might be learned that can be built into the lab process in the future. For the present, the physicians are pleased with results that arrive within 20 to 30 minutes.