Enhancing the industrial use of process capability studies

Abstract: The overall aim of this thesis is focused on how the industrial use of process capability studies can be improved. This aim is achieved by using two different approaches. As a first approach the gap between the theoretical frontier and practitioners use of process capability studies is identified, quantified, and explained. This is accomplished by using a series of questionnaire surveys within Swedish industry. Based on some retrospective case studies among nine Swedish organisations, a model how to implement process capability studies successfully, has been developed. A strategy to master variation by using several improvement methods together is presented using process capability studies as a framework within this strategy. The first four appended papers present the results of the first approach. As a second approach, the theoretical frontier of process capability studies has been developed to cover practical situations while also enhancing the possibilities of making proper decisions. Within the second approach a method how to use process capability studies when production runs are short has been developed and studied. Furthermore, the effect of skewness on estimates of process capability indices, which are numerical measures of process capability, has been studied. FFinally, since practitioners often seem to use process capability indices in inappropriate ways, a graphic approach for assessing, monitoring and improving the capability of processes has been put forward. The concept of process capability plots is introduced and two process capability plots are studied from both a theoretical and practical point of view in order to find out when each plot is most useful. The last three appended papers, which mainly are based on simulation studies, present the results of the second approach.