Sampling procedures and sample representativeness in a national telephone survey: A Portuguese example

Abstract

Objectives: This paper aims to assess the yielding of the sampling procedures and the representativeness of a sample evaluated in a national telephone survey in Portugal. Methods: The Portuguese telephone book (2007/2008) was the sampling frame for subjects >39 years (n = 1,934). The time spent with interviews and unsuccessful contacts was quantified and the yielding of the contact attempts compared according different schedules. Demographic and health characteristics of the participants were compared with those of the Portuguese population. Results: Among eligible individuals, the percentage of refusals was 40.4%, higher on Saturdays (45.2% vs. 30.7%) and evenings (45.2% vs. 36.7%). Evening interviews yielded a higher percentage of men (26.2% vs. 21.0%) and active workers (31.8% vs. 21.8%). Compared to the Portuguese population, our sample included a higher proportion of women (76.7% vs. 54.1%), elderly (>59 years: 61.4% vs. 46.2%) and more educated participants (>4 years: 38.3% vs. 26.3%). Sex- and age-specific estimates of smoking and obesity were similar to those in the Portuguese population. Conclusion: Despite the difficulties in reaching participants, this strategy may produce unbiased estimates for important health indicators. © 2009 Birkhäuser Verlag, Basel/Switzerland.