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.