Sampling and non-response bias on health-outcomes in surveys of the oldest old.

Surveys of the oldest old population are associated with several design issues. Place of residence and possible physical or cognitive impairments make it difficult to maintain a representative study population. Based on a Swedish nationally representative survey among individuals 77+, the present study analyze the potential bias of not using proxy interviews and excluding the institutionalized part of the population in surveys of the oldest old. The results show that compared to directly interviewed people living at home, institutionalized and proxy interviewed individuals were older, less educated and more likely to be female. They had more problems with health, mobility and ADL, and a significantly increased mortality risk. If the study had excluded the institutionalized part of the population and/or failed to use proxy interviews, the result would have been severely biased and resulted in underestimated prevalence rates for ADL, physical mobility and psychologic problems. This could not be compensated for weighting the data by age and sex. The results from this study imply that accurate population estimates require a representative study population, in which all individuals are included regardless of their living conditions, health status, and cognitive ability.