Response and non-response to a quality-of-life question on sexual life: a case study of the simple mean imputation method.

We investigated the non-response rates to the question "I am satisfied with my sex life" in the Functional Assessment of Cancer Therapy-General questionnaire in Chinese (n = 769), Malay (n = 41) and Indian (n = 33) patients in Singapore, a multi-ethnic society whose residents are said to have a conservative sexual attitude. Non-response rates to the question were 44%, 22% and 24% in the three groups respectively. The rates were much higher than that reported previously in a US study (7%) and used in the associated simulation study of the simple mean imputation method. We further examined the Chinese respondents in detail. The odds of non-response and the scores among the responders were associated with several demographic and clinical characteristics. Using the checklist proposed by Fayers et al. [Stat Med 1998; 17: 679-696] to assess the data patterns, we found that the application of the simple mean imputation is questionable. We employed an alternative (multiple) imputation procedure that took into account covariates that predicted the odds of non-response and the observed response scores. We compared the analytic results based on different approaches to handling missing values, and found that analysis based on the simple mean imputation gave results similar to that based on multiply imputed data even in this quite extreme example.