Where begins any proper psychological research? The right answer is – from carefully and diligently selected participants. Within the boundaries of psychological terminology, a population means a pool of people, each of them has some specific characteristics. This has nothing in common with the population of a town or a country. Predominantly, psychologists speak about a population of young women with dyslexia or a population of tower crane operators. A population, defined in this way may take into account a few features, such as all 4th-grade children, or a lot of parameters, such as tower crane operators over fifty who are color-blind. Obviously, a student coursework or an experiment in the psychological field of study should describe only correctly generalized populations. As long as we cannot survey all people in a chosen population, we rather survey a sample. If we were going to carry out a dissociative identity disorder case study, we, evidently, would not be able to survey all people with this disorder. Instead, while arranging an experiment, we choose a sample of people with dissociative identity disorder and then attempt to apply a proper generalization to all patients.
Given the above definition of population, we can say, that our target sample will be representative as long as individual participants of this sample are typical for the population. One can easily find out that inappropriateness of an unusual sample is a popular discursive subject for nursing papers topics. Nonetheless, when obtaining a representative sample does not appear feasible, an alternative experiment with a random sample could be devised. Random selection requires a completely different approach for generalization. However, as soon as a researcher begins the experiment, it is inevitable to make sure that he or she is free of systematic bias. Thus, a researcher may select participants, unconsciously preferring interesting surnames or some features of their appearance.
The second approach to resolving the problem of representative samples is the alternative that uses so-called opportunity samples. In this case, you are expected to work with volunteers exclusively, simply ignoring randomness or representativeness. Such type of study is popular in universities, where young psychology researchers choose their classmates and coevals as a sample; also, a school psychologist could test schoolchildren on a voluntary basis assigning to them a 4th grade book report. Sure thing, these participants would be nowhere near a representative sample, but at least their eagerness to help might underlie a pilot study with future prospects. Above all, your report should comprise the most comprehensive information about people under observation as well as about conditions of your experiment or survey.
There is not any exact answer to the question of obtaining participants. You might say – but I can't write my paper without a sample! – and you, of course, will be undoubtedly right. The point is, you should teach yourself independently how to lure the participants with words or money or other inducements. Depending on specifics of the chosen population, people might be interested if you will organize your experiment as a game or if you will promise to tell them results afterwards, emphasizing that these results may be relevant for their lives. In addition, some people are eager to assist provided they can make a contribution to science. Here is the list of promising sources that you can use to assemble an appropriate sample: