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SUBJECTS AND SAMPLING

Second, stratified sampling is used to ensure that an adequate

number of subjects is selected from different subgroups. For example, if a researcher is studying beginning elementary school teachers and be- lieves that there may be important differences between male and female teachers, using simple random or systematic sampling would probably not result in a sufficient number of male teachers to study the differ- ences. It would be in this situation tint to stratify the popula- tion of teachers into male and female teachers and then to select sub

necessary

jects

from each subgroup. The samples can be selected in one of two

ways. A proportional stratified sample, or

propwtiona2

allocation, is used

when the number of subjects selected from each stratum is based on the percentage of subjects in the population that have the characteristic used to form the stratum. Thus, in the previous example, if 5 percent of

the population of

elementary

teachers is male, 5 percent of the sample

would also be male teachers.

A second approach is to take the same number of subjects from

each stratum, regardless of the percentage of subjects from each stra- tum in the population. This method is used often because it ensures that a number of subjects will be selected from each stratum. For instance, if only 10 percent of a population of 200 elementary teach- ers are male, a proportional sample of 40 would include only 4 male teachers. To study male teachers it would be better to include all 20 male teachers in the population for the sample and randomly select 20 female teachers. This sampling procedure is referred to dispropor- tional because the number of subjects in the sample from each sub- group is not proportional to the percentage of the subgroups in the population. Disproportional stratified sampling is not limited to taking the same number of subjects from each subgroup. When dispropor- tional sampling is used the results of each stratum need to be weighted

sufEcient

as

to estimate values for the population

as

a whole.

In the following example disproportional stratified sampling en- sures that the same number of first and third graders are selected ran- domly.

months) were randomly selected.” 93)

and

2, age, 8 years, 8 :: .tasi;l988, p.

;

:

_ :

(Clements

Nas-‘.‘:,:: .: :

‘,.

__

.’ ,.;

.:::

Stratified random sampling is illustrated in Figure 4.1. In this ample the population is divided first into three different age groups,

ex-

89

I

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