During process development, it is always a debatable issue whether the variation in analytical results is due to the measurement system (MS) or due to the process. The best approach is to quantify total variation coming from the MS prior to any process improvement activity. This quantification is done by “Gage Reproducibility & Repeatability” (GRR). This article describes the usage of GRR for quantifying variation from various sources and selecting a suitable MS for the analysis. In this study, two instruments, a potentiometer and ultra high pressure liquid chromatography (UPLC), were evaluated for the assay measurement of a key starting material (KSM) supplied by a vendor. As a result of the GRR study, it was found that the potentiometer was not a suitable instrument, because of the high variation contributed by it, whereas UPLC was found to be suitable, because of the insignificant variation contributed by it towards the assay. In addition to this, it was also observed that the variation contributed by the KSM samples was insignificant, indicating that those samples were coming from a robust process, and the vendor was found to be suitable for supplying the KSM.