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.