Every engineering design is subject to variations that can arise from a variety of sources, including manufacturing operations, variations in material properties, and the operating environment. When variations are ignored, non robust designs can result, which are expensive to produce or fail in service. The concept of robust design may be first used by Taguchi. He introduced this concept to improve the quality of a product whose manufacturing process involves significant variability or noise~\cite{Taguchi93}. Robust design aims at minimizing the sensitivity of performances to variations without controlling the causes of these variations. First, we propose a new robustness index and compare it with three robustness indices used in the literature. Then, we develop a sequential tolerance synthesis method.