GR&R问题（(%PT是什么）

客户的问题：
Precision to tolerance ratio (%PT) is similar to the %GRR in your table. %GRR compares the precision of the measurement method to the product variation. %PT compares the precision of the measurement method to the process tolerance (+/- 1.5 à process tolerance is 3.0). The precision is the total variation, not including part variation but including appraiser variation and repeatability (I think this is the number you call R&R in your spreadsheet, but it needs to be calculated as the standard deviation, Sp). Then Precision to tolerance is 6*Sp / 3.0.
为什么我们客户的（%PT=6*Sp / 3.0），SP是怎么算的？
难道做GR&R还要计算standard deviation？实在不理解啊