SUBJECTS AND SAMPLING
too small. Most researchers use general rules of thumb in their studies, such as having at least 30 subjects for correlational research, and at least 15 subjects in each group in an experiment. In surveys that sample a
population, often a
small percentage of the population must be
sampled, for example, less than 5 or even 1 percent. Of course if the
sample is too small, it is likely that the results obtained cannot char- acterize the population. Formal statistical techniques can be applied to determine the number of subjects needed, but in most educational studies these techniques are not used.
In educational research a major consideration with sample size is concluding that a study with a relatively small sample that found no dif- ference or no relationship is true. For example, suppose that you are studying the relationship between creativity and intelligence and, with a sample of 20 students, found that there was no relationship. Is it rea- sonable to conclude that in reality there is no relationship? Probably not, since a probable reason for not finding a relationship is because such a small sample was used. In addition to the small number of sub
it is likely that there may not be many differences in either creativ-
ity or intelligence, and without such differences it is impossible to find that the two variables are related. That is, with a larger sample that has different creativity and intelligence scores, a relationship may exist. This problem, interpreting results that show no difference or relation- ship with small samples, is subtle but very important in educational re- search since so many studies have small samples. we will see in Chap- ter 9, it is also possible to misinterpret what is reported as a “significant” difference or relationship with a very large sample. Also, a sample that is not properly drawn from the population is misleading, no matter
what the size.
Sometimes subjects will be motivated to respond in certain ways. Clues for this phenomenon will be found in the description of how the sub jects were selected. For example, if a researcher was interested in study- ing the effectiveness of computer simulations in teaching science, one approach to the problem would be to interview teachers who used com- puter simulations. The researcher might even to select only those science teachers who had used the simulations more than two years. It is not hard to understand that the selected teachers, because they had been using the simulations, would be motivated to respond favorably ward them. The response would be consistent with the teachers’deci- sion to use simulations. Psychology students may be motivated to give in- accurate responses in studies conducted by their psychology professor if