Samples and Sampling

• The term "sampling," as used in research, refers to the process of selecting the individuals who will participate (e.g., be observed or questioned) in a research study.
• A sample is any part of a population of individuals on whom information is obtained. It may, for a variety of reasons, be different from the sample originally selected.

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 a 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.

Random Versus Nonrandom Sampling

• Sampling may be either random or nonrandom. Random sampling methods include simple random sampling, stratified random sampling, and cluster random sampling. Nonrandom sampling methods include systematic sampling, convenience sampling, and purposive sampling.

Random Sampling Methods

• A simple random sample is a sample selected from a population in such a manner that all members of the population have an equal chance of being selected.
• A stratified random sample is a sample selected so that certain characteristics are represented in the sample in the same proportion as they occur in the population.
• A cluster random sample is one obtained by using groups as the sampling unit rather than individuals.
• A two-stage random sample selects groups randomly and then chooses individuals randomly from these groups.
• A table of random numbers is a table of numbers, listed and arranged in no particular order, that is used to select a random sample.

Nonrandom Sampling Methods

• A systematic sample is a sample obtained by selecting every nth name in a population.
• A convenience sample is any group of individuals that is conveniently available to be studied.
• A purposive sample is a sample selected because the individuals have special qualifications of some sort, or because of prior evidence of representativeness.

Sample Size

• Samples should be as large as a researcher can obtain with a reasonable expenditure of time and energy. A recommended minimum number of subjects is 100 for a descriptive study, 50 for a correlational study, and 30 in each group for experimental and causal-comparative studies.

External Validity (Generalizability)

• The term "external validity," as used in research, refers to the extent that the results of a study can be generalized from a sample to a population.
• The term "population generalizability" refers to the extent to which the results of a study can be generalized to the intended population.
• The term "ecological generalizability" refers to the extent to which the results of a study can be generalized to conditions or settings other than those that prevailed in a particular study.

Replication

• When a study is replicated, it is repeated with a new sample and sometimes under new conditions.