# 88

## CHAPTER 4

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)

mothers

and SteVensQn,

# Systematic Sampling In systematic sampling every

nth

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

ex-

the

# 74Oth,

then the

# 1,24Oth,

and so on until 100 subjects were selected.

# Systematic sampling is virtually the same as simple random sampling. It

is certainly much

more

convenient.

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

cla.ssroom

is a cyclical pattern in the list (referred to

as ~tiodicit~).

If

every

nth sub

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

ject

population. Alphabetical lists do not usually create suitable for choosing subjects systematically.

periodicity

and are

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

as

results in

more

homogeneous groups, the sample will be

more

repre-

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.

member selected.

systematic tih

sampling: the population is

of

Every

## Stratified

sampling: selected from Strata

## Subjects are

or

groups

of

the population.