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　Sampling methods

　　Data are often collected from a sample rather than from a population. If the whole population is examined, the survey is called a census.

　　There are two types of sampling methods:

　　Probability sampling method – is a sampling method in which there is a known chance of each member of the population appearing in the sample.

　　- Random

　　- Stratified random

　　- Systematic

　　- Multistage

　　- Cluster

　　If random sampling is used, it is necessary to construct a sampling frame.

　　A sampling frame should have following characteristics

　　- completeness

　　- accuracy

　　- adequacy

　　- up to dateness

　　- convenience

　　- non-duplication

　　Non probability sampling method – is a sampling method in which the chance of each member of the population appearing in the sample is not know, i.e. quota sampling.

　　Example 1:

　　The following statements relate to which different types of data

　　(i). Secondary data are data collected especially for a specific purpose

　　(ii). Discrete data can take on any value.

　　(iii) Qualitative data are data that cannot be measured

　　(iv). Population data are data arising as a result of investigating a group of people or

　　Objects.

　　Which of the statements are true?

　　A.(i)and(ii)only

　　B.(ii)and(iii)only

　　C.(ii)and(iv)only

　　D.(iii)and(iv)only

　　Solution is D

　　It is primary data that is collected for a specific purpose so (i) is false. Continuous data can take on any value so (ii) is false. Both (iii) and (iv) are true.

　　Example 2:

　　Which of the following statements are not true?

　　I If a sample is selected using random sampling, it will be free from bias.

　　II A sampling frame is a numbered list of all items in a sample.

　　III In cluster sampling there is very little potential for bias.

　　IV In quota sampling, investigators are told to interview all the people they meet

　　Up to a certain quota.

　　A I, II, III and IV

　　B I, II and III

　　C II and III

　　D II only

　　Solution is C

　　A sampling frame is a numbered list of all items in a population (not a sample)

　　Cluster sampling involves selecting one definable subsection of the population which therefore makes the potential for bias considerable.