(X)OVER-BAR-CONTROL AND R-CONTROL CHARTS FOR SKEWED POPULATIONS

This paper proposes a heuristic method based on a weighted variance co ncept of setting up control limits of (X) over bar and R charts for sk ewed populations. It provides asymmetric control limits in accordance with the direction and degree of skewness estimated from the sample da ta, by using different variances in computing upper and lower control limits. For symmetric populations, however, these control limits are e quivalent to those of Shewhart control charts. The new heuristic contr ol charts are compared by Monte Carlo simulation with Shewhart charts and the geometric control charts of Ferrell. When the underlying popul ation is Weibull or Burr's, the heuristic charts are found to perform better than Shewhart or geometric charts as the skewness increases.