Why can PPS sampling select the sample before the complete totality is obtained?

Because PPS sampling can first accumulate the existing items for the system options, the newly acquired or the rest of the items can be sequentially ranked later, does not affect the previous process of selecting the sample. For example, for the overall size of 10,000 items for sampling, if only 9000 of them to complete the sorting work, this time, although not all of the 10,000 items to complete the sorting, but PPS sampling can be the first of these 9,000 ranked for the sample selection, that is, to be able to get the whole before the start of the selection of samples.