Statistical process control with vary- parameter based on average product length

Aimed at the limitation that current statistical process control method are effective to only one fixed extend shift, a kind of robust control chart is proposed and the average product length calculating method of control chart is also discussed in this paper. Then, an optimal design scheme of the robust control chart is discussed. Comparisons of average product length of the robust optimal design with those of different charts show that the method proposed in our paper is effective to shifts of different sizes.