A note on “Monitoring the ratio of two normal variables using variable sampling interval exponentially weighted moving average control charts”

Considered monitoring of the ratio of two correlated normal random variables based on an approximation. In this note, we derive a closed form expression for the ratio of two correlated normal random variables. We point out that the approximation used in Nguyen et al. does not always perform well.     

A study on the improvements of new product development procedure performance–an application of design for Six Sigma in a semi-conductor equipment manufacturer

With the pressure of intensive global competition and the trend of shortening product life-cycle, the procedure for new product development (NPD) has become the focus of business concerns and investment of large corporations and the key strategy to upgrade competitiveness of corporations. An effective NPD procedure can meet the demands on quality, time-to-delivery and cost limitations of a corporation. Measuring and analysing the performance of the existing NPD procedure and pinpointing and resolving the problems of the existing NPD procedure is the key to improve the NPD performance to meet a corporation's demands. Six Sigma is a systemic procedure commonly used to improve product quality. This paper proposes a model to evaluate and improve the performance of NPD procedures by following the systemic procedure of Six Sigma and applying criteria defined by application of performance matrix, factor analysis, and theory of constraints. The resulting model is then verified with real life NPD procedures from a certain semi-conductor equipment manufacturer such that corporative competitiveness can be enhanced through the elevating cycle of performance evaluation, problem excavation, causes analysis, and countermeasure determination and execution proposed by this paper.