Session 2 Sources of data

　　Main contents:

　　1. Types of data

　　2. Secondary data

　　3. Sampling methods

　　2.1 Types of data

　　Data can be classified as follows:

　　(a)Primary and secondary data

　　Primary data – are data collected especially for a specific purpose.

　　Secondary data – are data which have been collected elsewhere, for some

　　other purposes, but which can be used or adapted for the

　　survey being conducted.

　　(b)Discrete and continuous data

　　Discrete data – are data which can only be taken on a finite or countable

　　number of values within a given range.

　　Continuous data – are data which can take on any value.

　　(c )Sample and population data

　　Sample data – are data arising as a result of investigating a sample

　　Population data – are data arising as a result of investigating the population.

　　2.2 Secondary data

　　The main sources of secondary data are: governments; banks; newspaper; trade journals; information bureau; consultancies; libraries and information services.

　　- It is essential to believe that the secondary data used is accurate and reliable.

　　2.3 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.