The research must choose a sample that is most representative of the study population (the larger phenomenon or group to which we wish to generalize). When a sample is drawn out of convenience (nonprobability sample), rationale and limitations must be clearly provided. To do so, you must outline the characteristics of the population (by gender, race/ethnicity, socioeconomic status, or other relevant characteristics of the sample) and take this in consideration in choosing the sample.
The key reason for being concerned with sampling is that of validity, the extent to which the interpretations of the results of the study follow from the study itself and the extent to which results may be generalized to other situations with other people or situations. Sampling is critical to external validity, the extent to which findings of a study can be generalized to people or situations other than those observed in the study.
To generalize validly, the findings from a sample to some defined population requires that the sample has been drawn from that population according to one of several probability sampling plans. By a probability sample is meant that the probability of inclusion in the sample of any element in the population must be given a priori. All probability samples involve the idea of random sampling at some stage. In experimentation, two distinct steps are involved:
• Random selection: Participants to be included in the sample have been chosen at random from the same population. Define the population and indicate the sampling plan in detail.
• Random assignment: Participants for the sample have been assigned at random to the experimental and control conditions.
