This paper proposes a control chart with variable sampling intervals (VSI) to detect increases in the expected value of the number of defects in a random sample of constant size n the upper one-sided c-VSI chart.
The performance of this chart is evaluated by means of the average time to signal (ATS). The comparisons made between the standard FSI (fixed sampling intervals) and the VSI upper one-sided c - charts indicate that using variable sampling intervals can substantially reduce the average time to signal. Using stochastic ordering we prove that this reduction always occurs.
Special attention is given to the choice of the proposed control chart parameters and to the chart graphical display.