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, cluster random sampling, and
two-stage 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 lists and arranges numbers in no particular order and can
be used to select a random sample.
NONRANDOM SAMPLING METHODS
• A systematic sample is obtained by selecting every n th name in a population.
• A convenience sample is any group of individuals that is conveniently available to
be studied.
• A purposive sample consists of individuals who have special qualifi cations of some
sort or are deemed representative on the basis of prior evidence.
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
• 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, cluster random sampling, and
two-stage 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 lists and arranges numbers in no particular order and can
be used to select a random sample.
NONRANDOM SAMPLING METHODS
• A systematic sample is obtained by selecting every n th name in a population.
• A convenience sample is any group of individuals that is conveniently available to
be studied.
• A purposive sample consists of individuals who have special qualifi cations of some
sort or are deemed representative on the basis of prior evidence.
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
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