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
from each subgroup. The samples can be selected in one of two
ways. A proportional stratified sample, or
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
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
to estimate values for the population
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)
2, age, 8 years, 8 :: .tasi;l988, p.
Nas-‘.‘:,:: .: :
Stratified random sampling is illustrated in Figure 4.1. In this ample the population is divided first into three different age groups,