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know how my constituents feel about the issue.” Unfortunately, many of these samples are virtually
without merit. We do not know what population (whom) they represent.

These and other examples can be used to illustrate the point. Observations at airports may
overrepresent high-income groups, whereas observations taken at bus terminals may overrepresent low-
income groups. Surveys taken at a rock concert would likely be different from those taken at a symphony

concert. In the case of political attitudes, we do know that many special interest groups make it a matter
of policy to write letters to their political representatives. A thousand people vitally concerned about an

issue may write more letters than a million people who are indifferent. Polls taken on the Internet have
become more popular and suffer from the same drawbacks. The point we are making is this: Because the
population from which the sample came is unknown, it is unclear to whom the data can be generalized.

We can generalize to known populations, but only with some risk. We will have more to say about this
below.
The examples used here are extreme and the problems obvious, but there are instances where these

problems are not as serious or as apparent. In these instances, some researchers believe that convenience
sampling is a good alternative to random sampling.
As noted earlier, most laboratory research in psychology, human and nonhuman, uses a convenience

sampling procedure. Some universities require that students taking the introductory psychology course
serve as participants in research projects of their choosing. When participants are required to participate in

research and are allowed to choose certain experiments over others, then for any given experiment, it is
simply impossible to specify the population to which the sample data can be generalized. In other words,
to what individuals, other than those of the sample, are the data relevant? We have here a sample in

search of a population.
Requiring students to participate in research serves several purposes. It assures that each student has
an opportunity to learn firsthand about scientific research. In this regard, an attempt is made to make

participation in research a worthwhile educational experience. It also assures that participants are
generally available for research, thus serving the purposes of the researcher and that of psychology as a
science. The system requiring participants to participate in research of their choosing operates in the

following manner: Research projects to be undertaken are listed on a bulletin board (or a Web site), with a
brief description of the project and a sign-up sheet indicating the time, place, and experimenter. If our

earlier description was clear, you will recognize this as a convenience sampling procedure. Although the
students are required to participate in research, they choose the particular project in which to participate.
If students are available at a given time, and the particular experiment appeals to them, they simply sign

their name on the sign-up sheet.
Frequently, the description on the sign-up sheet is neutral, but sometimes it is not. The titles alone—

for example, Reaction Time to Electrical Stimulation, Problem Solving and Cognitive Skills, or