Abstract
In one of the first Statistical Sidebars I wrote (published in the March–April 2015 issue of the Journal of Visual Impairment & Blindness), readers were introduced to types of sampling. The point was made that when conducting research in the field of blindness, researchers often make use of convenience sampling. A convenience sample occurs when participants who fit a study’s criteria are enrolled in the study, sometimes by simply going to a location that is likely to have a large number of people who are blind (e.g., to a state school for blind students or a convention of a blindness organization). The main drawback of convenience sampling is that the study results lack generalizability due to the bias of the sample. As such, a study that uses convenience sampling is somewhat caught between a single-subject approach (where a treatment or intervention is used with very few participants to focus on the efficacy of that treatment or intervention) and a randomized control group approach (where a sample of participants are randomly chosen from a large population and randomly assigned to treatment and control groups). The use of a larger sample through convenience sampling, rather than utilizing a single-subject approach, allows for slightly greater generalization, but the lack of random selection of participants means that selection bias restricts large-scale generalization of the findings.
In the current issue, the author of the article entitled, “Locus of Control in College Students with and Without Visual Impairments, and the Visual Characteristics That Affect It,” used convenience sampling to gather participants who are blind for the study. He put together a sample of 15 participants who are blind. This sample is larger than a single-subject approach, thus, it has the potential to show that any experimental effects are more generalizable than they would be if the author had studied just one individual. The convenience sampling procedure the author used, however, drastically limits generalizability of study effects, since all participants were college students from one college in Iran. In addition, the study’s subjects were college students who happened to visit their college’s center for students who are blind on a particular Sunday. It would be a stretch to generalize the study effects from such a sample to a much larger group, given the particulars of place, time, and circumstance.
To try to limit the drawbacks resulting from convenience sampling, these authors also hand-picked a set of participants with sight who matched the blind participants on the variables of gender, education, and age (variables that previous research showed might be particularly impactful on the variables of interest of the current study). By creating this matched group, the authors were better able to isolate the relationship of blindness with the study’s variable of interest. In essence, the author embraced the limitations of convenience sampling and designed the study to specifically mitigate those limitations, as much as he was able.
The take-home point is that although the “gold standard” for research designs that study groups is to use a randomly drawn sample and randomly assign participants to treatment and control groups, it is generally not possible to gather such large groups in the field of visual impairment because of the fact that vision loss is a low-incidence disability. Instead, researchers in the field need to determine what are the limitations of small sample sizes, conveniently drawn samples, and heterogeneous groups and then use study designs, data structures, and statistical analyses that address those limitations.