A single control chart for monitoring the frequency and magnitude of an event

While many control charts have been developed for monitoring the time interval (T) between the occurrences of an event; many other charts are employed to examine the magnitude (X) of the event. This article presents a single statistical process control (SPC) chart (called the rate chart) for simultaneously monitoring the time interval T and magnitude X based on the ratio between X and T. The new chart is more effective than an individual T chart, individual X chart, or a combined T&X chart for detecting the out-of-control status of the event, especially for detecting downward shifts (sparse occurrence and/or small magnitude). More profound is that, comparing with an individual T or X chart, the rate chart performs more uniformly for detecting both T shift and X shift, as well as the joint shift in T and X. The rate chart has demonstrated its potential not only for manufacturing systems, but also for non-manufacturing sectors such as supply chains, office administration and health care industry.