SUBJECTS AND SAMPLING
In many research designs it is either unfeasible or unnecessary to obtain a probability sample. In these situations a nonprobability sample is used.
sample is one in which the probability of including
population elements is unknown. Usually, not
element in the pop-
Probabil- ity of selection not known.
ulation has a chance of being selected. It is also quite common for the population to be the same as the sample, in which case there is no im- mediate need to generalize to a larger population. In fact you will find that much of the educational research reported in journals, especially
experimental studies, uses a group of from a larger population.
that has not been selected
Convenience Sampling A convenience sample is a group of subjects se- lected because of availability, for example, a university class of a profes- sor conducting some research on college students, classrooms of teach- ers enrolled in a graduate class, schools of principals in a workshop, people who decide to go to the mall on Saturday, or people who re- spond to an advertisement for subjects. There is no precise way of gen- eralizing from a convenience sample to a population. Also, the nature of the convenience sample may bias the results. For example, if the available sample for studying the impact of college is the group of alumni who return on alumni day, their responses would probably be quite different from those of all alumni. Similarly, research on effective teaching that depends on teachers in a particular geographic area, be- cause they are available, may result in different findings than research
Convenience sample: available sample.
done in other geographic areas. Although we need to be very
of convenience samples, often
this is the only type of sampling possible, and the primary purpose of the research may not be to generalize but to better understand relation- ships that may exist. Suppose a researcher is investigating the relation- ship between creativity and intelligence, and the only available sample is
a single elementary school. The study is completed, and the
cate a moderate relationship: Children who have higher intelligence tend to be more creative than children with lower intelligence. Because there was no probability sampling, should we ignore the findings or sug- gest that the results are not valid or credible? That decision seems overly harsh. It is more reasonable to interpret the results as valid for children similar to those studied. For example, if the school serves a low socio- economic area, the results will not be as useful as those from a school that all socioeconomic levels. The decision is not to dismiss the findings but to limit them to the type of subjects in the sample. more and more research accumulates with different convenience samples, the
overall credibility of the results is enhanced.