LIFE-STYLE SURVEYS - THE COMPLETE ANSWER

Study objectives - These were as follows: to study incompleteness of d ata, herein called item non-response, generated by a self completion q uestionnaire; to identify the characteristics of item non-responders a nd the types of questions liable to high item non-response rates; and to discuss possible reasons for item non-response. Design - Item non-r esponse patterns in 12 307 responders (62%) to a representative postal survey based on a stratified sample drawn from family health services authorities' (FHSA) registers were investigated. Main outcome measure s - Data were analysed for item non-response in three groups depending on when the questionnaire was returned (wave analysis). The overall c ompletion rate of the questionnaire was examined and the natural logar ithm of the proportion of completed questions was used as an outcome v ariable in multiple regression analysis. Item non-response to key ques tions and questions of different types was examined. Results - Wave an alysis: the overall completion rate of the questionnaire was 86% in qu estionnaires returned before the first reminder and 83%-84% in those s ent back after subsequent reminders. Overall pattern of item non-respo nse: respondents failed to complete a mean of 15% and a median of 10% of the questionnaire. All questions in the questionnaire had some item non-response, ranging from 1% to 85%. Completion rates were associate d with gender, age, indicators of lower socioeconomic status, and gene ral health status. Individual questions: particular types of questions were liable to have higher item non-response, for example, linked bin ary questions. Conclusions - Item non-response in population postal su rveys is likely to present problems in the interpretation of data by i ntroducing bias additional to that of total non-response. Item non-res ponse does not increase greatly with later returns, suggesting that th e quality of data across responses generated by two reminders is simil ar. There are obstacles to reducing item non-response, such as respond ent error or socioeconomic and health characteristics of the general p opulation, that cannot be totally overcome. However, the evidence that individuals tend to complete only options within questions that apply to them and their positive behaviour is useful information for those designing questionnaires and interpreting survey data.