NONRESPONSE BIAS IN EC RESPIRATORY HEALTH SURVEY IN ITALY

In the three Italian centres involved in the European Community Respir atory Health Survey (ECRHS), prevalence of asthma-like symptoms was as sessed through a mailback questionnaire, Since the nonresponse rate wa s not negligible, ranging 10-18%, we investigated whether nonresponse bias affected the results and, if so, whether the bias could be elimin ated from the final estimates of prevalence. A screening questionnaire was sent by mail to 7,000 randomly selected subjects 20-44 yrs of age , and nonresponders were contacted again by phone, Additional informat ion was collected on a subsample of the respondents through a clinical interview. A logistic regression analysis showed that, except for one symptom (awakening for coughing), symptom prevalence significantly de creased from the first to the subsequent contact, when controlling for age, sex, centre and season of interview, The decrease in symptom pre valence was largely independent of smoking habits and socioeconomic st atus, and was seemingly caused by a symptom-related self-selection, Wh en correcting results according to a linear regression model, observed estimates appeared to be slightly overestimated, by 4-10%, A simulati on with the Italian data showed that the bias increased steeply at non response rate higher than 30%, a situation quite common in asthma Surv eys. In conclusion, nonresponse bias affects the results of ECRHS in I taly, slightly inflating prevalence estimates, To make reliable compar isons on international data in the presence of different nonresponse r ates, a correction of the observed prevalence seems necessary.