Comparable ethical issues arise with prospective longitudinal studies, except that the researcher is required to anticipate the demands at reassessment as if subjects were to be approached for the first time. Here, the importance of advance permissions to track subjects as they migrate geographically and socially cannot be overstated. In this context, it is useful to anticipate difficulties, retaining in longitudinal studies those who may be or become most disadvantaged or whose health may fail. Discussions with experienced ethicists are often helpful. Sometimes guidance can improve the research design and compliance because the changes in research procedures sought to obtain ethical approval are so often well focused. In aging research, when community-dwelling populations are to be sampled, there is a particular problem with refusals, and this will raise specific ethical issues. First, on recruitment there is a bias toward under-representation of the disadvantaged in society. Individuals unable or unwilling to take part for reasons of mental illness or impairment are a major cause for concern as these may be those same individuals for whom the research findings may be most relevant. Individuals who are aware of the health risks posed by their lifestyle are also less likely to volunteer to take part and, when they do, may fail to return for follow-up assessments. Solutions to these problems are imperfect, and most researchers seek to identify the source and size of the errors accrued. Good research practice goes a long way to overcoming subject resistance. So, if a personal approach is taken with each volunteer and, for a group that is anticipated to have problems with research, extra steps are introduced. These are often labor-intensive and demand continuing personal contact with a named person, familiar with the reservations expressed by the volunteer and careful to explain all procedures.
The approach taken to resolve these issues can be coercive or rely on inducements. Ethicists may vary in where a boundary can be set between encouragement to participate and a subject being disadvantaged in some way if they refuse or withdraw consent. Among subjects who find great difficulty responding helpfully to requests from professional people, there are sometimes those who can see why, as individuals, they should take part. Where appropriate, these individuals can be encouraged to seek the involvement of their peers. The success of this snow balling approach is tempered by the need to explain—sometimes repeatedly—the purpose of the research, its possible benefits to the wider public, and the nature and inconvenience their taking part might lead to.
Noncompliance causes huge heartaches and headaches. Carefully judged continuity of personal contacts between the volunteer subject and single named research worker can achieve high levels of compliance, but often this is detected only upon study completion. Some measures of reliability and validity need to be part of the study design. In self-reports of food intake, for example, at least three elements of reliability are built into the measurement instruments. These are basically estimates of consistency: Does the subject respond in the same direction when the same or similar question is posed? From test occasion to occasion, is the same question answered in the same way? Is there evidence of inconsistency attributable to some confounder (e.g., poor memory)? Validity is much more problematical and is addressed in a later section.
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