604.2021SAMPLING.4.ppt
- 1. 1 SAMPLING What is a sample? Samples and populations Defining the population Target vs accessible populations Random vs nonrandom sampling Random sampling methods Nonrandom sampling methods Sample size External validity
- 2. 2 What is a sample? The term sampling, as used in research, refers to the process of selecting individuals who will participate in a research study. A sample is any part of a population of individuals on whom information is obtained.
- 3. 3 Samples and Populations The term population, as used in research, refers to all the members of a particular group. It is the group of interest to the researcher, the group to whom the researcher would like to generalize the results of the study. A target population is the actual population to whom the researcher would like to generalize; the accessible population is the population to whom the researcher is entitled to generalize. A representative sample is a sample that is similar to the population on all characteristics.
- 4. 4 Samples and Populations A sample in research study is the group on which information is obtained. The larger group to which one hopes to apply the results is called the population. For example: All students at ELT Department majoring in ELT constitute a population, 50 of those students constitute a sample.
- 5. 5 Defining the population The first task in selecting a sample is to define the population of interest. The population, in other words, is the group of interest to the researcher to whom he/she would like to generalize the results of the study. For example: All students attending a department All students at a university All secondary school principals in a country All English language teachers at a university
- 6. 6 Defining the population The actual population called the target population to whom the researcher would like to generalize is rarely available. The population to which a researcher is able to generalize is the accessible population. The first choice is ideal choice, the second choice is the researcher’s realistic choice.
- 7. 7 Defining the population Ex: Research problem to be investigated: The effects of computer-assisted instruction on the reading achievement of freshmen at universities in a country. Target population: All freshmen at universities in a country Accessible population: All freshmen at a university Sample: Ten percent of freshmen at that university
- 8. 8 Random vs Nonrandom Sampling Sampling may be either random or nonrandom. Random sampling methods include: – Simple random sampling – Stratified random sampling – Cluster random sampling – Two-stage random sampling
- 9. 9 Random Sampling A simple random sampling is one in which every member of the population has an equal and independent chance of being selected. (Example: Using a table of random numbers.) Stratified random sampling is a process in which certain groups contain subgroups, or strata, are selected in the same proportion as they exist in the population.
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- 11. 11 Random Sampling Two-stage random sampling combines cluster random sampling and simple random sampling. Rather than selecting 100 students from 3000 students of ninth-graders located in 100 classes, Stage 1: Select 25 classes randomly from 100 classes Stage 2: Select 4 students from each class This method is less time consuming than visiting most of the 100 classes. – Cluster random sampling is used to ensure that the sample is representativeon other variables as well.
- 13. 13 Nonrandom Sampling Nonrandom sampling methods include: – Systematic sampling – Convenience sampling – Purposive sampling
- 14. 14 Nonrandom Sampling Systematic sampling: In systematic sampling every nth individual in the population list is selected for inclusion in the sample. A random start (starting with nth individual) The sampling interval (distance in the list between each of the individuals selected for the sample) The sampling ratio (population size/desired sample size) 5000/500= .10 or 10 percent Periodicity (If the population has been ordered systematically, it may lead to a markedly biased sample
- 15. 15 Nonrandom Sampling Convenience sampling: A convenience sample is a group of individuals who (conveniently) are available for the study. In general, convenience samples cannot be considered representative of any population and should be avoided if possible. The study should be replicated with a number of similar samples to decrease the likelihood that the results obtained were simply a one-time occurrence.
- 16. 16 Nonrandom Sampling Purposive sampling: On occasion, based on previous knowledge of population and the specific purpose of the research, investigators use personal judgment to select a sample.
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- 18. 18 External Validity & Replication External validity: It refers to the extent the results of a study can be generalized from a sample to a population. Replication: When a study is replicated, it is repeated with a new sample and sometimes under new conditions.
- 19. 19 Assignment Research Problem: “Communicative approach is more effective tha direct method in EFL classes at secondary high schools” Please statethe following sampling statements: 1. Target population: 2. Accessible population:
- 20. 20 Assignment 3. Simple random sampling: 4. Cluster random sampling: 5. Stratified random sampling: 6. Two-stage random sampling: 7. Convenience sampling: 8. Purposive sampling: 9. Systematic sampling:
- 21. 21 THANK YOU FOR LISTENING!