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Nonrandom sampling (1)

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Adibah H. Mutalib

The document discusses different sampling methods: systematic sampling selects every nth individual from a population list to reduce selection bias. Convenience sampling relies on readily available individuals, but the samples may not represent the overall population. Purposive sampling allows the researcher to use judgment to select a sample suitable for the research purpose. The key challenges are avoiding periodicity in systematic sampling and having sufficient knowledge of the population for purposive sampling.

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Systematic Convenience Purposive Population Genaralizability Ecological Genearalizability
Every nth individual in the population list is selected. eg: the principal of a school has 1000 students, she wants to know how students feel about the new menu at cafeteria, so the principal:- 1. get a list of the students name (in alphabetical order). 2. she select every 10th student until she has a sample of 100 students to be interviewed.
PERIODICITY-a marked bias sample caused by the arrangement pattern of individual on the list accidentally coincides with the sampling interval. eg: grouped by gpa(grade point average), high/low interval: only good/poor graded get chosen. Therefore, researchers should carefully examine the list and avoid bias.
CONVENIENCE SAMPLING A convenience sample is a group of individuals who conveniently available. samples: 1. first 50 people who walk in. 2. interview people at downtown. 3. two front rows students.
Bias- 1. not downtown = not interviewed 2. unwilling = not interviewed 3. willing = strong opinion 4. interview time = at work Ingeneral, convenience sample cannot be considered representative of any population and should be avoided.
PURPOSIVE SAMPLING Based on previous knowledge of a population and the specific purpose of the research,researcher use personal judgement to select a sample. eg:SUITABLE SAMPLE a. 2 good students,2 average & 2 weak b. sample from Retired Workers Association eg: sample know the target A. people in charge of school B. people with experience THUS, the only challenge with purposive sampling is the researchers previous knowledge must be thorough.
Sample should be as large as a researcher can obtain with a reasonable expenditure of time and energy. Ideals; 100 samples for DESCRIPTIVE STUDY 50 samples for CORRELATION STUDY 30 samples in each group for EXPERIMENTAL STUDY& CAUSAL-COMPARATIVE STUDY
External Validity = the result of a study can be generalised from sample to population. Ecological generalizability = result of a study can be generalised to other settings. Population generalizability = result of study can be generalised to the intended population -representativeness: relevant characteristics Overlooked method-random student=random teacher=random result. Lost subject effect representativeness, researcher who lost 10 % sample are advised to acknowledge this limitation.
1. researcher should describe the sample thoroughly; reader judge the result validity. 2. Replication; repeat the study on different group or situation. If result is the same; generalise it.

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Nonrandom sampling (1)

  • 1. Systematic Convenience Purposive Population Genaralizability Ecological Genearalizability
  • 2. Every nth individual in the population list is selected. eg: the principal of a school has 1000 students, she wants to know how students feel about the new menu at cafeteria, so the principal:- 1. get a list of the students name (in alphabetical order). 2. she select every 10th student until she has a sample of 100 students to be interviewed.
  • 3. PERIODICITY-a marked bias sample caused by the arrangement pattern of individual on the list accidentally coincides with the sampling interval. eg: grouped by gpa(grade point average), high/low interval: only good/poor graded get chosen. Therefore, researchers should carefully examine the list and avoid bias.
  • 4. CONVENIENCE SAMPLING A convenience sample is a group of individuals who conveniently available. samples: 1. first 50 people who walk in. 2. interview people at downtown. 3. two front rows students.
  • 5. Bias- 1. not downtown = not interviewed 2. unwilling = not interviewed 3. willing = strong opinion 4. interview time = at work Ingeneral, convenience sample cannot be considered representative of any population and should be avoided.
  • 6. PURPOSIVE SAMPLING Based on previous knowledge of a population and the specific purpose of the research,researcher use personal judgement to select a sample. eg:SUITABLE SAMPLE a. 2 good students,2 average & 2 weak b. sample from Retired Workers Association eg: sample know the target A. people in charge of school B. people with experience THUS, the only challenge with purposive sampling is the researchers previous knowledge must be thorough.
  • 7. Sample should be as large as a researcher can obtain with a reasonable expenditure of time and energy. Ideals; 100 samples for DESCRIPTIVE STUDY 50 samples for CORRELATION STUDY 30 samples in each group for EXPERIMENTAL STUDY& CAUSAL-COMPARATIVE STUDY
  • 8. External Validity = the result of a study can be generalised from sample to population. Ecological generalizability = result of a study can be generalised to other settings. Population generalizability = result of study can be generalised to the intended population -representativeness: relevant characteristics Overlooked method-random student=random teacher=random result. Lost subject effect representativeness, researcher who lost 10 % sample are advised to acknowledge this limitation.
  • 9. 1. researcher should describe the sample thoroughly; reader judge the result validity. 2. Replication; repeat the study on different group or situation. If result is the same; generalise it.