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Volunteer Samples

Random sampling of broad populations is possible for survey research in which slight demands are made on the subjects. For example, most public-opinion polls are able to obtain random samples since they typically ask only a few questions and take only a few minutes of the respondent's time. Demands on the subject are much greater in most educational research; consequently, even if the researcher selects a random sample he can rarely get cooperation from all the subjects selected. (You will recall that when some subjects refuse to participate in a study, the remaining subjects no longer constitute a random sample because persons who agree to participate are likely to be different from those who do not.) For educational studies that employ other methods than survey, such as correlational or experimental research, the demands on the subject are usually much greater and consequently it is virtually impossible to obtain the cooperation of all subjects selected by random sampling.

Furthermore, as you learned in chapter 3, legal and ethical constraints on the researcher require him to obtain informed consent from human subjects (or their parents in the case of minors) before involving them in a research project.
As a result of the aforementioned conditions, nearly all educational research must be conducted with volunteer subjects. We know that volunteer subjects are likely to be a biased sample of the target population since volunteers have been found in many studies to differ from nonvolunteers.
There is a considerable body of research on the characteristics of volunteers. Rosenthal and Rosnow have conducted an excellent review of research in this area and have identified a number of characteristics that have been found to occur in studies of volunteer subjects.''
Conclusions about distinguishing characteristics are listed at four levels of confidence depending on the accumulation of research evidence that supports each conclusion. Within each category the conclusions are listed in order starting with those having the strongest evidence supporting them.
 
Conclusions Warranting Maximum Confidence
 
  • Volunteers tend to be better educated than nonvolunteers, especially when personal contact between investigator and respondent is not required.
  • Volunteers tend to have higher social-class status than nonvolunteers, especially when social class is defined by respondents' own status rather than by parental status.
  • Volunteers tend to be more intelligent than nonvolunteers when volunteering is for research in general but not when volunteering is for somewhat less typical types of research such as hypnosis, sensory isolation, sex research, small-group and personality research.
  • Volunteers tend to be higher in need for social approval than non-volunteers.
  • Volunteers tend to be more sociable than nonvolunteers.
 
Conclusions Warranting Considerable Confidence
 
  • Volunteers tend to be more arousal-seeking than nonvolunteers, especially when volunteering is for studies of stress, sensory isolation, and hypnosis.
  • Volunteers tend to be more unconventional than nonvolunteers, espedally when volunteering is for studies of sex behavior.
  • Females are more likely than males to volunteer for research in general, but less likely than males to volunteer for physically and emotionally stressful research (e.g., electric shock, high temperature, sensory deprivation, interviews about sex behavior).
  • Volunteers tend to be less authoritarian than nonvolunteers.
  • Jews are more likely to volunteer than Protestants, and Protestants are more likely to volunteer than Roman Catholics.
  • Volunteers tend to be less conforming than nonvolunteers when volunteering is for research in general but not when subjects are female and the task is relatively "dinical" (e.g., hypnosis, sleep, or counseling research).
 
Conclusions Warranting Some Confidence
 
  • Volunteers tend to be from smaller towns than nonvolunteers, especially when volunteering is for questionnaire studies.
  • Volunteers tend to be more interested in religion than nonvolunteers, especially when volunteering is for questionnaire studies.
  • Volunteers tend to be more altruistic than nonvolunteers.
  • Volunteers tend to be more self-disclosing than nonvolunteers.
  • Volunteers tend to be more maladjusted than nonvolunteers, especially when volunteering is for potentially unusual situations (e.g., drugs, hypnosis, high temperature, or vaguely described experiments) or for medical research employing clinical rather than psychometric definitions of psychopathology.
  • Volunteers tend to be younger than nonvolunteers, especially when volunteering is for laboratory research and especially if they are female.
 
Conclusions Warranting Minimum Confidence
 
  • Volunteers tend to be higher in need for achievement than nonvolunteers, especially among American samples.
  • Volunteers are more likely to be married than nonvolunteers, especially when volunteering is for studies requiring no personal contact between investigator and respondent.
  • Firstborns are more likely than laterborns to volunteer, especially when recruitment is personal and when the research requires group interaction and a low level of stress.
  • Volunteers tend to be more anxious than nonvolunteers, especially when volunteering is for standard, nonstressful tasks and especially if they are college students.
  • Volunteers tend to be more extraverted than nonvolunteers when interaction with others is required by the nature of the research.
 
The degree to which these characteristics of volunteer samples affect research results depends on the specific nature of the investigation. For example, a study of the level of intelligence of successful workers in different occupations would probably yield spuriously high results if volunteer subjects were studied, since volunteers tend to be more intelligent than nonvolunteers. On the other hand, in a study concerned with the cooperative behavior of adults in work-group situations, the tendency for volunteers to be more intelligent may have no effect on the results, but the tendency for volunteers to be more sociable could have a significant effect. It is apparent that the use of volunteers in research greatly complicates the interpretation of research results and their generalizabilitv to the target population, which includes many individuals who would not volunteer.
The work of Rosenthal and Rosnow provides the researcher with valuable information that should be considered carefully when planning a study in which volunteer subjects are to be used. The researcher can review the characteristics listed above and for each ask such questions as: How relevant is this characteristic to the dependent and independent variables to be employed in my study? If relevant, how would the difference between volunteers and nonvolunteers on this characteristic be likely to influence the research results? Are any data available on my target population for checking whether these characteristics are present among the volunteers I will employ as subjects? The first two questions often can be answered on the basis of knowledge about the variables in his study that the researcher gains during a review of previous research. Graduate students can also get help on these questions by discussing them with members of their research committees and other faculty members who have worked in the student's area of interest.
With regard to the third question, Rosenthal and Rosnow suggest two methods, which they refer to as exhaustive and nonexhaustiue. In the exhaustive approach, all potential subjects are compared on as many relevant variables as possible in which volunteers and nonvolunteers may differ. For example, suppose the investigator plans to ask for volunteers from a sophomore general psychology class to participate in a study of dient responses to different interview techniques used in counseling, such as directive versus nondirective counseling. Among the characteristics identified by Rosenthal and Rosnow, differences between volunteers and nonvolunteers on such variables as need for social approval, conformity, and authoritarianism may lead to differences in the responses of volunteers and nonvolunteers in the planned research. The researcher could first measure all students on these variables by administering appropriate tests in the class, could then call for volunteers, and then compare the scores of volunteers and nonvolunteers to determine how the groups differ on these variables. When working with student samples the investigator may not need to administer measures on all relevant variables. Most schools and colleges routinely administer batteries of tests to all students; these data can be drawn upon for comparisons between volunteers and nonvolunteers.
In the nonexhaustive method, data on nonvolunteers are not available, but data are available oh subjects who differ in their willingness to volunteer. For example, the investigator can compare the scores of easy-to-recruit volunteers and hard-to-recruit volunteers on some critical variable such as intelligence and then extrapolate to obtain an estimate of the intelligence of nonvolunteers in the target population. Easy-to-recruit volunteers may be those who will volunteer repeatedly over a period of time, those who will volunteer without being offered an incentive such as pay, or those who respond more promptly to requests for volunteers (such as newspaper advertisements or letters).
These procedures for obtaining information about characteristics of a specific volunteer sample, when combined with the general characteristics of volunteers described by Rosenthal and Rosnow, are of great assistance to the researcher in interpreting findings and generalizing to nonvolunteer populations. In reviewing research on the volunteer subject, Rosenthal and Rosnow also identified ten situational variables that tend to increase or decrease the rates of volunteering. These findings form the basis for the following suggestions for increasing the rate of volunteering and thus reducing volunteer bias:
 
Suggestions for Improving Rate of Volunteering
 
  • Make the appeal for volunteers as interesting as possible, keeping in mind the nature of the target population.
  • Make the appeal for volunteers as nonthreatening as possible so that potential volunteers will not be "put off" by unwarranted fears of unfavorable evaluation.
  • Explicitly state the theoretical and practical importance of the research for which volunteering is requested.
  • Explicitly state in what way the target population is particularly relevant to the research being conducted and the responsibility of potential volunteers to participate in research that has potential for benefiting others.
  • When possible, potential volunteers should be offered not only pay for participation but small courtesy gifts simply for taking time to consider whether they will want to participate.
  • Have the request for volunteering made by a person of status as high as possible, and preferably by a woman.
  • When possible, avoid research tasks that may be psychologically or biologically stressful.
  • When possible, communicate the normative nature of the volunteering response (i.e., volunteering is the normal thing to do).
  • After a target population has been defined, an effort should be made to have someone known to that population make the appeal for volunteers. The request for volunteers itself may be more successful if a personalized appeal is made.
  • In situations where volunteering is regarded by the target population as normative, conditions of public commitment to volunteer may be more successful; where nonvolunteering is regarded as normative, conditions of private commitment may be more successful.
  • Most of these suggestions will not only increase the rate of volunteering, but will also reduce attrition in both volunteer and nonvolunteer samples. The graduate student should consider these suggestions very carefully since following them will reduce volunteer bias and also result in better research planning.