Industry requires effective evaluation methods for quality control of automobile parts and bearings. The ISO standard defines the calculation of roughness parameters from material ratio curve ("MRC") and material ratio curve on normal probability paper ("MPC") as effective methods for evaluating surfaces with excellent lubrication and frictional characteristics. ISO 4287 specifies the slice method as a calculation method for MRC. The analysis time of the slice method is lengthy owing to the large amount of calculation. Therefore, ISO 21920-2 specifies the sort method as a calculation method for MRC. The Sort method significantly reduces analysis time owing to the small amount of calculation. A previous study revealed that errors occurred in the MRC by the sort method compared to the slice method. However, the previous study concluded that the errors were acceptable compared to the time cost. In addition, a plateau surface is a surface with excellent sliding property. The roughness parameters of a plateau surface have to be calculated from MPC. However, in the case of expression on normal probability paper, the difference between MRCs calculated using the sort and slice methods increases as both ends approach. Therefore, the roughness parameters calculated by each MPC are expected to be different results. This study reports the affect that increasing differences have on the roughness parameters. We aim to contribute to the establishment of a highly effective evaluation method by verifying the validity of calculating MPC by the sort method.