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Nonparticipation and withdrawal from a study after initial enrollment pose a threat to epidemiologic research on late-life cognitive disorders. Resulting biases may arise in several ways. Persons who agree to participate in a study may do so because of their own perception of risk. For example, persons with a family history of dementia may be more likely to enroll in research studies of cognitive impairment (1, 2). Alternatively, persons who lead a healthy lifestyle may choose to participate in such studies as a reflection of their quest for novelty and stimulation. On the other hand, persons with incipient or overt cognitive impairment may be fearful of participating or may shun involvement in anything unfamiliar. The cognitive status of nonparticipants in recent studies of dementia incidence is unknown (3–7). However, 2 older investigations that were able to determine the cognitive status of initial nonparticipants (8, 9) demonstrated that the nonparticipants had lower cognitive test scores than participants. Little is known about future cognitive outcomes in persons who refuse participation in studies of cognitive aging.

The Mayo Clinic Study of Aging (MCSA) was initiated in 2004 to study the natural history of cognitive impairment in a geographically defined population (10). Over 2,000 nondemented persons were enrolled in the initial cohort. We observed differences between persons who agreed to participate and those who refused (10), but at the time, we had no information about differences in cognitive status among nonparticipants. In the 7 years since the completion of cohort enrollment, we have had the opportunity to use the medical record linkage system of the Rochester Epidemiology Project (11–13) to investigate the prevalence and incidence of dementia among nonparticipants. We report here the incidence rates for dementia from the MCSA, having previously reported on the prevalence (14) and incidence (15) of mild cognitive impairment as well as on the progression from mild cognitive impairment to dementia (16).

Incidence of Dementia Among Participants and Nonparticipants in a Longitudinal Study of Cognitive Aging

Re: “Incidence of Dementia Among Participants and Nonparticipants in a Longitudinal Study of Cognitive Aging”

The concepts that case-referent studies provide for the estimation of "relative risk" only if the illness is "rare", and that the rates and risks themselves are inestimable, are overly superficial and restrictve. The ratio of incidence densities (forces of morbidity)-and thereby the instantaneous risk-ratio-is estimable without any rarity-assumption. Long-term risk-ratio can be computed through the coupling of case-referent data on exposure rates for various age-categories with estimates, possibly from the study itself, or the corresponding age-specific incidence-densities for the exposed and nonexposed combined-but again, no rarity-assumption is involved. Such data also provide for the assessment of exposure-specific absolute incidence-rates and risks. Point estimation of the various parameters can be based on simple relationships among them, and in interval estimation it is sufficient simply to couple the point estimate with the value of the chi square statistic used in significance testing.

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Estimability and estimation in case-referent studies

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References

Incidence of Dementia Among Participants and Nonparticipants in a Longitudinal Study of Cognitive Aging

Re: “Incidence of Dementia Among Participants and Nonparticipants in a Longitudinal Study of Cognitive Aging”

Estimability and estimation in case-referent studies

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