M-charts for failure frequency
We have a production process with very low (ppm-ish) fail rates that we would like to monitor for fails. Rather than monitoring ppm fail rates, it is a stronger test to monitor the number of fails (or time) between them, under the assumption that they follow a binomial distribution. In an M chart (this is what we call it) you then calculate a Z score every time there is a fail and chart as usual....