Simple random sampling is illustrated in the following study of mothers’strategies for influencing their children’s schooling.
provided by the principal of the school.” (Baker 1986, p. 157)
Systematic Sampling In systematic sampling every
element is se-
lected from a list of all elements in the population, beginning with a randomly selected element. Thus, if there is a need to select 100 sub- jects from a population of 50,000, every nth element would correspond to every 500th subject. The first element is selected randomly. In this ample that would be some number between 1 and 500. Suppose 240 were randomly selected as a starting point. The first subject chosen for the sample would be the 240th name on a list, the next subject would be
and so on until 100 subjects were selected.
Systematic sampling is virtually the same as simple random sampling. It
is certainly much
There is a possible weakness in systematic sampling if the list of cases in the population is arranged in a systematic pattern. For instance, if a list of fourth-graders in a school division is arranged by and students in the classrooms are listed from high to low ability, there
is a cyclical pattern in the list (referred to
that is selected corresponds to the pattern, the sample would repre- sent only a certain level of ability and would not be representative of the
population. Alphabetical lists do not usually create suitable for choosing subjects systematically.
Stratified Sampling A modification of either simple random or system- atic sampling is first to divide the population into homogeneous sub- groups and then select subjects from each subgroup, using simple ran- dom or systematic procedures, rather than the population as a whole. This is termed stratified sampling. The strata are the subgroups. Strati- fied sampling is used primarily for two reasons. First, as long the sub- groups are identified by a variable related to the dependent variable in the research (e.g., socioeconomic status in a study of achievement) and
homogeneous groups, the sample will be
sentative of the population than if taken from the population as a whole. This result reduces error and means that a smaller sample can be chosen.
sampling: the population is
sampling: selected from Strata