Telephone survey: post-stratification adjustments to compensate non-coverage bias in city of Rio Branco, Northern Brazil

OBJECTIVE: To evaluate the effects of using post-stratification weight to correct the bias due to low coverage of households with telephones. METHODS: A Comparison was made of results collected by the Household Survey with those of the VIGITEL (Telephone Survey to Monitor Risk and Protective Factors for Chronic Diseases) in Rio Branco, Northern Brazil, in 2007 whose coverage was 40% of landline phones. The potential bias in the VIGITEL survey was expressed by the difference between the rates of prevalence of the VIGITEL and Household Survey, calculated as the square root mean square error (MSE) as a measure of the accuracy of the estimate. RESULTS: The weighting procedure of VIGITEL corrected potential bias in the prevalence of consumption of fruit and vegetables, meat with visible fat, smoking, bad self-assessment of health status and morbidity of cholesterol or triglycerides. In the prevalence of physical activity in leisure time and morbidity of asthma, bronchial asthma, chronic bronchitis or emphysema, the procedure adopted by VIGITEL did not reduce the potential bias. CONCLUSIONS: in order to construct post-stratification weights which minimize the potential bias in estimates of the variables due to low coverage of households with landlines, it becomes necessary to use alternative methods of weighting and strategies of selecting external variables.