Properties of the Markov Dependent Attribute Control Chart with Estimated Parameters
The performance of attribute control charts that monitor Markov-dependent data is usually evaluated under the assumption of known process parameters, that is, known values of a the probability an item is nonconforming given the previous item is conforming and b the probability an item is conforming given the previous item is nonconforming. In practice, these parameters are usually not known and are calculated from an in-control Phase I-data set. In this paper, a comparison of the in-control ARL (average run length) properties of the attribute chart for Markov-dependent data with known and estimated parameters is presented. The probability distribution of the estimators is developed and used to calculate the in-control ARL and standard deviation of the run length of the chart with estimated parameters. For particular values of a and b, the in-control ARL values of the charts with estimated parameters may be very different than those with known parameters. The size of the Phase-I data set needed for charts with estimated parameters to exhibit the same in-control ARL properties as those with known parameters may vary widely depending on the parameters of the process, but in general, large samples are needed to obtain accurate estimates. As the Phase-I sample size increases, the in-control ARL values of the charts with estimated parameters approach that of the known parameter case but not in a monotonic fashion as in the case of the X-bar chart.