Sampling methods
　　· Random sampling 随机抽样
　　· Stratified random sampling 分层抽样
　　· Systematic sampling 系统抽样
　　· Multistage sampling 多级抽样
　　· Cluster sampling 集群采样
　　Random sampling 随机抽样
　　Random sampling means every item in the population has an equal chance of being selected.
　　Stratified random sampling 分层抽样
　　This involves dividing the total population into strata or categories (for example age groups) and then taking random samples from each of the strata or categories.
　　Systematic sampling 系统抽样
　　Systematic sampling is a sampling method which works by selecting every nth item after a random start. The gap is the sampling interval.
　　Multistage sampling 多级抽样
　　Multistage sampling involves dividing the population into a number of sub-populations and then selecting a small sample of these sub-populations at random.
　　Cluster sampling (non-random) 集群采样
　　· Cluster sampling involves splitting the population into convenient groups and then selecting a number of groups at random. Every item in the sample is then investigated.
　　· This is a non-random sampling method.
　　Quota sampling 配额抽样
　　In quota sampling， randomness is forfeited in the interests of cheapness and administrative simplicity. Investigators are told to interview all the people they meet up to a certain quota.