Sampling Techniques

               SAMPLING TECHNIQUES

by Daniel R. Tomal in Action Research for Educators. Rowman & Littlefield Education.2010.pp29.

Sampling is defined as selecting the subjects for a study from a specific population. There are several sampling techniques used in research, such as simple random sampling, tratified sampling, cluster sampling, systematic sampling, convenience sampling, and purposeful sampling.

1.Random Sampling
Random sampling provides a process in which each subject has an equal and independent chance of being selected. Simple random sampling is picking names from a hat. Random sampling is considered one of the best ways to select subjects from a defined population for a study because it provides a high probability that the subjects will represent the entire population.

2. Stratified Sampling
Stratified sampling is a method of selecting equally sized subgroups of subjects from a known population. For example, if a researcher desired to gain the opinion from both male and female teachers, then he or she would select equally sized proportions of subjects from each group. If there were 100 male teachers and 50 female teachers, and the researcher desired an equal representation of 30 subjects, he or she might select 20 male teachers and 10 female teachers.

3. Cluster Sampling
Cluster sampling is concerned with selecting random groups versus individuals, such as classes of students. For example, if a researcher desired 90 subjects, he or she might select three classes of 30 students from the population. Cluster sampling is useful for securing subjects from large populations or when there are intact groups, such as class-rooms, and it is not practical or possible to select individual students. It can also be a convenient and quick process for selecting subjects.

4. Systematic Sampling
Systematic sampling is rarely used in education. This sampling process involves selecting every preset number of people from a list (e.g., every tenth person). The formula is expressed by K the number in population desired divided by the desired sample number. This expression results in a number representing K, such as K 10, and, therefore, every tenth name would be selected. Systematic sampling is similar to the process used in physical education class whereby the coach calls out each student by number, and every tenth student is placed on a specific team. Systematic sampling is sometimes criticized for not being a truly random selection process since all the subjects do not have an equal and independent chance of being selected, and certain subgroups could be excluded. This can be reduced by ensuring that the subjects are randomly listed before selecting the subjects.

5. Convenience Sampling
Convenience sampling is used as the term implies—the subjects are conveniently accessible. For example, a teacher may use his or her own class of students for the study. This type of sampling is not random, but in action research, the researcher is generally not concerned with the generalizability of the results. Generalizability refers to the extent towhich the findings can be applied to the general population. Also, while statistical methods can be applied to data obtained by convenience sampling, the results are more valid and reliable when random sampling has been used for selecting the subjects for the study.

6. Purposeful Sampling
Purposeful sampling is a commonly used sampling technique in action research. Purposeful sampling selects those subjects who are the specific individuals for whom improvement is desired. In action research, the researcher generally has a defined target population in which he or she desires improvement, and, therefore, purposeful sampling is often the method of sampling. Purposeful sampling is also a popular method used in qualitative research because, similar to action research, the researcher is concerned with selecting “information rich” subjects and often has a predetermined target population identified.