CHAPTER 4: SELECTING A SAMPLE

• Identify and describe four random sampling techniques.

• Select a random sample using a table of random numbers.

• Identify three variables that can be stratified.

• Select stratified samples, cluster samples, and systematic samples.

• Identify and describe three nonrandom sampling techniques.

• Identify and briefly describe two major sources of sample bias.

• Describe quantitative and qualitative sampling strategies.

Sampling-

• the process of selecting a number of participants for a study in such a way that they represent the larger group from which they were selected

Sample-

• comprises the individuals, items, or events selected from a larger group referred to as a population

• If a quantitative sample is selected, the research results on it will be generalizable to the population.

• If a qualitative sample is selected, it will likely focus on a limited group of articulate participants

See example on the bottom of page 102.

• -500 teachers were selected to discuss what they felt about teacher unions

• -Conclusions based on intervies

• -How about 500 teachers who are members of the NEA?

• -There are several relatively simple sampling techniques that could be applied to represent a sample of teachers

Defining a Population

• Population-

• is the group of interest to the researcher, the group to which the results of the study will ideally generalize.

• Ex.—all 10th grade students, all elementary school gifted children in Utah

Defining a Population

• Target population-

• the population that the researcher would ideally like to generalize to

• Available (accessible) population-

• the population that the researcher can realistically select from

• -It is important to define your population in sufficient detail so that others may determine how applicable your findings are to their situation.

• -Regardless of what sampling approach is used, it should describe the characteristics of the sample. (i.e.- the number of participants, the demographics, percentage of each racial and gender group, level of education, achievement level)

Selecting a Random Sample

• -Very important step in conducting a research study for quantitative research

• -Selecting a representative sample is not a haphazard process

The steps are same:• Identify the problem

• Determine the required sample size

• Select the sample

• Four basic random sampling techniques:

– simple random sampling

– stratified sampling

– cluster sampling

– systematic sampling

– Probability sampling-

• it is possible for the researcher to specify the probability, or chance, that each member of a defined population will be selected for the sample.

Steps in Simple Random Sampling

– Selecting a sample involves the following steps:

• An Example of Random Sampling

Stratified Sampling

• Subgroup or strata- a variable that can be divided into groups

• Steps for Equal-Sized Groups in Stratified Sampling

• An Example of Proportional Stratified Sampling

Cluster Sampling

• Cluster- any population where we find an intact group of similar characteristics

Systematic Sampling

• Steps in Systematic Sampling

Determining Sample Size

• (see table 4.2) Samples sizes (S) required for given populations (N)

• Avoiding Sampling Error and Bias

Selecting a nonrandom sample

• Nonprobability or nonrandom sample

• Convenience sampling

• Purposive sampling

• Quota sampling

Qualitative Sampling: Definition and Purpose

Task # 4: Performance Criteria