This paper discusses the mechanism and reason for decreasing the handling polynomial function data on difference with durum by datum.From the error theory,some exemples are analysed and the equivalent principle for the intervals on datum by datum is presented.The optimal number for intervals is analysed in detail and shown by some examples for the correct result.At last,by making comparison between some methods for handling the physical experimental data to show the advantages of them,the proper condition and range in use for every method is also presented.