Non-Probability Sampling Methods

Non-Probability Sampling Methods

• Non-probability sampling methods do not rely on probability to select a sample from the population.
• There are four types of non-probability sampling methods:
  • Convenience Sampling
  • Judgment Sampling
  • Referral Sampling
  • Quota Sampling

Convenience Sampling

• Convenience samples are drawn at the convenience of the interviewer.
• The most convenient areas to a researcher are often "high traffic" areas such as shopping or busy pedestrian areas.
• The selection of the place and prospective respondents is subjective rather than objective.
• Certain members of the population are automatically eliminated from the sampling process.

Judgment Sampling

• Judgment samples require the judgment or an "educated guess" as who should represent the population.
• Often, the researcher or an individual with knowledge about the population chooses those individuals they feel constitute the sample.
• Judgment sampling is often used in focus group studies.

Referral Sampling

• Referral sampling, also known as "snow ball samples", requires respondents to provide the names of additional respondents.
• The sample list begins with a short list of sample units that is smaller than the total sample desired.
• Referral sampling is most appropriate when there is a limited and disappointingly short sample frame and when respondents can provide the names of others who would qualify for the survey.

Quota Sampling

• Quota sampling establishes a specific quota for various types of individuals to be interviewed.
• The quotas are determined through the application of the research objectives and are defined by key characteristics used to identify the population.
• A field worker is provided with screening criteria that classify the potential respondent into a particular quota cell.

Strengths and Weaknesses of Sampling Methods

• Developing a sample plan involves a logical sequence of steps to draw and ultimately arrive at the final sample.
• The sample plan may focus on optimizing costs, precision, or both precision and costs.

5-Stage (Multi Stage) Sampling

• 5-stage sampling is a further development of the principle of cluster sampling.
• It is used in large-scale surveys for a more comprehensive investigation.
• The researcher may use two, three, or four stage sampling, depending on the complexity of the study.
• Multi-stage sampling is comparatively convenient, less time-consuming, and less expensive, but may introduce element sample bias due to the unequal size of some of the selected sub-samples.