The process standard deviation of charts does not need to be estimated in setting up the control limits for tracking mean differences and thus the estimation errors in the process standard However, most of the proposed control charts in the literature assumed the shift size to be known, whereby in reality the exact Therefore, this paper proposed the used o measure the performance of variable sampling interval EWMA t (VSI EWMA t) chart when the shift size could not be specifie demonstrated and the results are compa shift, VSI EWMA t chart outperforms the t chart, the VSI t chart and the EWMA t chart