15.4.1  Purposeful sampling

Supposing you wanted to focus on a limited number of respondents who you had selected because you think their in-depth information will give good insight into an issue that little is known about. This would be a qualitative research study, and the appropriate method to use would be what is called purposeful sampling. Purposeful sampling involves the selection of a sample of individuals with a particular purpose in mind. Using purposeful sampling you would select subjects for specific reasons, such as:

  • They meet particular criteria of interest in your research, e.g. very poor compliers with anti-TB treatment; well-nourished children; women who use depo-provera for family planning, etc.
  • They show wide variations in their knowledge, attitudes or practice to a particular health issue, e.g. towards people living with HIV, or towards FGM or early marriage.
  • They have particular knowledge or expertise, e.g. traditional birth attendants or herbalists in your community.

Purposeful sampling can be very informative. However, this sampling method cannot produce results that can be generalised to the population as a whole, and it may be difficult to avoid personal bias or preference when you are selecting your sample. For example, supposing you want to investigate the use of particular herbs which are taken for pain relief in your kebele. You might decide to interview people who you know to be traditional herbalists. However, this might introduce a personal bias as you would only be able to ask those traditional herbalists who are known to you. Moreover, there may be herbs which are widely used in the community, but that are not recommended by the traditional herbalists. You would get a more accurate picture of the use of pain-relieving herbs in the community if you questioned a wider range of subjects – not just the herbalists.

15.4  Non-probability sampling methods

15.4.2  Quota sampling