According to Darius Singpurwalla (2013), there are two ways sampling include: sampling probability (random) sampling and probability (not random). In particular, there are 3 probability sampling doing including random sampling, stratified sampling and sampling systems.
According to Nguyen Hai Shui (2013). Stratified random sampling is done by dividing the overall success of the organization under one or more criteria, criteria related to research purposes (such as broken businesses by region, by region, by type , by size, ...). Then in each group, using a simple random sampling or sampling system to pick out of the sample units. For stratified sampling, the number of units in each group selected may follow the proportion of units it occupies in the overall structure, or can not comply with the ratios.
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