Validity of data collected by telephone survey: a comparison of VIGITEL 2008 and 'Saúde em Beagá' survey.

Objective: To assess the validity of the estimates obtained through telephone survey and to measure the impact of the post-stratification weighting factor to adjust estimates.

Methods: The same questionnaire was completed by two independent samples of the population living in the municipality of Belo Horizonte city (Barreiro and West regions). One sample (n=440) completed the questionnaire of VIGITEL 2008 (telephone survey), and the other (n=4,048) of Saúde em Beagá (face to face household interview). The results of the two samples for 18 health-related variables were compared by means of test statistics. At first, residents who had a landline telephone line were compared to those who reported not having a telephone line; then, VIGITEL estimates, with and without post-stratification weight, were compared with Saúde em Beagá estimates.

Results: Subjects who owned a landline telephone line had indicators for better economic conditions (housing, schooling, and skin color); higher prevalence of chronic diseases; lower exposure to risk factors for chronic diseases; and improved access to health services, compared to the those who reported not having a telephone line. Most VIGITEL estimates (without post-stratification weight) were similar to the sample of Saúde em Beagá that reported owning a residential landline, showing no major impact of the methodology to obtain this data (lower information bias). Even without post-stratification weight, VIGITEL estimates were similar to those of Saúde em Beagá. With post-stratification weight, the estimates of "number of residents", "skin color" and "physical activity" did not differ from those obtained by the face to face survey.

Conclusion: The results of both surveys were very similar. Because of the lower cost, the telephone interview is a good option in public health for the behavioral risk-factor surveillance system.