Statistics
Some of my colleagues have recently been pushing multivariate charts. These charts combine several different process outcome values into a single number that’s placed on a chart. Unfortunately, this theoretically elegant approach breaks down in practice for reasons that will be explained here.…
Walter Shewhart’s average and range chart had been around for more than a decade when, in 1941, W. J. Jennett came up with the chart for individual values and moving ranges (an XmR chart). The XmR is built on the same foundation as the average and range chart, but it involves two unique ideas.…
The Weibull distribution is a continuous probability distribution often used in reliability analysis to model time-to-failure and product lifespans.
It has applications in engineering, medicine, energy, and insurance. It’s also good at describing survival statistics, such as…
Many erroneously think of a process behavior chart as a technique for monitoring a process. Consequently, they consider other monitoring techniques as alternatives to a process behavior chart. One of these proposed alternatives is an exponentially weighted moving average (EWMA). While an EWMA…
In the 1980s, demand for SPC classes outstripped the supply of competent instructors. Novices were teaching neophytes, and misinformation could be found everywhere. Out of this chaos, many incorrect ideas about process behavior charts became widely circulated. These ideas continue to be spread…
The way we think about our process will shape the way we collect, analyze, and interpret our data when things go wrong. This in turn will shape the actions taken and the results obtained. In this column, we look at an example of the difference between the traditional approach and an alternate…