Effects of age and cognitive function on data quality of standardized surveys in nursing home populations.

Surveys are adequate for residents with no/mild cognitive impairment but data quality is threatened in residents with moderate impairments. Precision and validity of responses from NHR with progressed cognitive impairment are potentially limited and results may be biased. The results clearly do support the need for a multidisciplinary 'general theory' of the question-/answer-process which has to be also inclusive for cognitively impaired elderly persons.