I am an undergraduate currently studying Quality engineering. My question is as follows: Performance to Tolerance (P/T) ratio uses σ_R&R / (USL - LSL) as an estimate of a gauge capability while GRR method uses σ_R&R / σ_total where σ_total is given by sqrt[(σ_R&R )^2 + (σ_product) ^2]. Which estimate/method (P/T ratio or GRR) gives a more meaningful estimate of the performance of a gauge? As an extension, which is more stringent? Any responses appreciated.