Suppose that a co-worker has been using Shewhart control charts to analyze a process but has only been using the upper and lower control limits to determine out-of-control conditions in the form of extreme points, without any consideration of the zone rules, runs and other statistical signals for the Xbar chart. When questioned about this, he responds that we only need to worry about the extreme points, since these are the ones that will indicate problems with lack of conformance of the parts to the specifications. How would you respond to his argument?