It is well documented that the distribution of the ratio between two random normal variables is asymmetric. As a consequence, the two-sided Shewhart control chart monitoring this ratio (denoted as Shewhart-RZ chart) has an average run length (ARL)−biased property. In order to overcome this drawback, we propose in this paper designing two separated one-sided control charts. Furthermore, we improve the efficiency of the Shewhart-RZ chart by combining the She-whart chart with a variable sampling interval strategy. The chart performance is then evaluated by using an average time to signal. The numerical results show that our proposed charts outperform the two-sided Shewhart-RZ control chart in detecting process shifts.