Question：A company wishes to carry out a national survey of adults' reading habits. To reduce travelling costs, the country was divided into constituencies. A sample of 50 constituencies was selected at random. With each of these constituencies, 5 polling districts were selected, again using random techniques. Interviewers will visit a random selection of 30 people on the electoral register of each district selected.

　　What sampling method is the company using?

　　A. Stratified

　　B. Systematic

　　C. Multi-stage

　　D. Simple random

　　The correct answer is: C

　　The sample is selected in stages, at first selecting not individuals but constituencies.

　　Multi-stage sampling is the correct answer as this method involves dividing the population into a number of sub-populations and then selecting a small sample of these sub-populations at random. Each sub-population is then divided further.

　　Stratified sampling involves dividing the population into strata and then taking a random sample from each stratum.

　　Random sampling is where every member of the population has an equal chance of being selected and systematic where every nth item after a random start is selected.