To assess the reliability of responses to questionnaires regarding previous illness, family history of cancer, and smoking and drinking habits, we repeated questionnaire surveys four times at intervals of 2 weeks and 1 year (short-term), and 4.5 years (long-term) among 440 subjects aged 40-69. The reliability was assessed using kappa statistic. Kappa was calculated both for complete data and data including missing values. The changes of mode of pre-after paired responses were also investigated. Our results from complete data showed both short- and long-term reliabilities of replies regarding smoking or drinking were excellent (mean kappa 0.85-0.99). The reliability of previous illness was excellent except for stroke for short-term intervals (mean kappa 0.85 -1.00), but varied depending on the kinds of illness with long-term intervals (mean kappa -0.01-0.75). Responses to family history had fair to excellent short-term reliability (mean kappa 0.54-0.85). Inclusion of missing value as an independent category reduced reliability remarkably. Subjects stating absence of medical history were more likely to have missing values for this item than subjects with some history. In conclusion, the reliability for information given on previous illness was as good as that on smoking and drinking for a short interval, but was lower for a long-interval probably due to the development of new cases. The reliability of a family history on cancer was slightly poorer than that of individual's previous cancer or other illnesses and that of smoking and drinking even for a short interval.
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