Home landline telephone coverage and potential bias in epidemiological surveys

MÉTODOS: Foram utilizadas as bases de dados da Pesquisa Nacional por Amostra de Domicílios no período de 1998 a 2003 para a estimativa das taxas de cobertura de linhas telefônicas residenciais nas cinco regiões geográfi cas brasileiras. Utilizou-se a regressão logística múltipla para identifi car os fatores associados à posse de linha telefônica fi xa. O impacto do vício nos intervalos com 95% de confi ança foi avaliado em função da precisão alcançada em cada situação.


INTRODUCTION
In Brazil, researchers have shown a growing interest in residential landline telephone use to perform epidemiological studies.a In general, population surveys use subscriber telephone directories to select samples of homes with home telephones.Among the problems resulting from this practice is the lack of updated information, causing the need, in the majority of studies, to compensate this with a search for telephone lines via internet, available on the corresponding telephone company's website.However, the response rates obtained (over 60%) stimulate the use of this strategy to conduct more time and cost effi cient surveys, enabling a reduction in interview costs and deadlines.
In Brazil, the Ministry of Health Department of Health Surveillance (SVS/ MS) has been developing the Sistema de Vigilância de Fatores de Risco para a Scheuren F. What is a survey?American Statistical Association.Washington D.C.; 2004 [cited 2009 Mar 20].Available from: http://www.whatisasurvey.infoa This survey is conducted annually, enabling changes in the Brazilian population's habits and behavior to be followed.However, it is necessary to analyze specifi c methodologies for questions such as: coverage, response rate and efficiency (related to post-stratifi cation weights to correct bias in the estimates of variables of interest).
The present study aimed to estimate the effect of home landline telephone coverage rates on potential information bias in epidemiological surveys.

METHODS
Databases from the Pesquisa Nacional por Amostra de Domicílios (PNAD -National Household Sample Survey) of the Instituto Brasileiro de Geografi a e Estatística (IBGE -Brazilian Institute of Geography and Statistics) were used between 1998-9 and 2001-3 to estimate home landline telephone coverage rates in Brazil.b This survey is conducted annually using a process of random selection of a probabilistic sample of homes on the national level, except for the Northern region's rural area.The PNAD adopts the Plano Complexo de Amostragem (PCA -Complex Sampling Plan), which involves the geographic stratifi cation of conglomerates with one, two or three selection stages. 7e Brazilian territory was divided into 36 strata, comprised as follows: 17 in the federation units (states of Rondônia, Acre, Amazonas, Roraima, Amapá, Tocantins, Maranhão, Piauí, Rio Grande do Norte, Pará, Alagoas, Sergipe, Espírito Santo, Santa Catarina, Mato Grosso, Mato Grosso do Sul and Goiás) and the capital and the Federal District comprise 18 independent strata; and the remaining federation units (states of Pará, Ceará, Pernambuco, Bahia, Minas Gerais, Rio de Janeiro, São Paulo, Paraná and Rio Grande do Sul) form two strata, one with the metropolitan regions (nine strata) and another with the remaining cities (nine strata), thus comprising the other 18 strata.The variables that defi ned the sampling plan design were considered in the analysis, known as stratum, primary sampling unit and sampling weight.Statistical analyses were performed with Stata software, version 8.0, where the SVY module, suitable for obtaining unbiased estimates with data coming from complex sampling plans, was used.
Home landline telephone coverage rate for each year defined was estimated using the expression where y hαβ is the variable's observable value (1 if owning a landline telephone, and 0 if not) of the β home, of the α primary sampling unit; of the h stratum; x hαβ equals 1for all β homes observed in the α primary sampling unit, of the h stratum; , Each home's weight is obtained from each h stratum's inverted sampling fraction.
Confi dence intervals for coverage rates were estimated using the expression , where As regards home landline telephone user profi le, in each geographic region, interviewees aged 18 or older were previously divided into two mutually exclusive groups, containing only those living in homes with or without a home landline telephone.By using the multiple logistic regression analysis, c where π(x) is the home landline telephone access probability according to the (x i ) variables, eight sociodemographic characteristics were defi ned (Table 1).Criteria for the fi nal inclusion of these variables in the model followed the stepwise technique, considering the 5% signifi cance level.
To estimate potential bias associated with lack of home landline telephone coverage, 2003 PNAD data, which brought health characteristics as an additional theme, were used to obtain prevalence estimates of chronic diseases in the population aged 40 years or older, living in the areas studied.Thus, interviewees were separated into two strata: those who own a home landline telephone and those who do not own it.Individuals who had the diagnosis of at least one of the following conditions confi rmed by a doctor or health professional were considered to have chronic diseases: spine or back diseases, arthritis or rheumatism, diabetes, bronchitis or asthma, heart diseases or hypertension.
Potential bias due to low coverage rates was estimated using the expression where the numerator is the absolute value calculated for the bias resulting from the difference between the prevalences of both strata, weighted by the proportion of people living in homes without a landline telephone who comprised the second stratum.Values above 0.40 for relative bias changed the 95% level initially established for confidence intervals (Cochran 1997), 1 which may cause wrong inferences to be drawn.

RESULTS
Results obtained showed a 50% growth in the number of homes covered by landline telephones in Brazil, between 1998 and 2003 (Figure 1).However, this change did not occur uniformly in the country.In the Northern, Northeast and Central-west regions, proportions of home landline telephones increased 26%, 36% and 22% respectively, whereas in the Southeast and Southern regions, they increased 58% and 56% respectively.Despite this growth, in 2003, the average rate of homes covered by landline telephones was 50.3%, showing that the Northern (41%) and Northeast regions (39%) were below average, unlike the Southeast (60%), Southern (63%) and Central-west regions (55%).
In addition, the proportion of adults living in homes without landline telephones was greater in rural areas and non-metropolitan regions (Table 1).Adults who reported not to be white showed greater proportion of lack of landline telephone access than self-reported white people.The number of adults without a home landline telephone was inversely proportional to the number of years of schooling.
Estimates obtained with the adjusted regression model pointed to associations among home landline telephone access, sociodemographic characteristics and geographic region ( access.Figure 2 shows the graphs with probabilities of owning a home landline telephone according to the ethnicity and level of education variables.The probability of owning a home landline telephone among white adults with fewer than fi ve years of schooling was found to be below 50%, whereas self-reported non-white adults needed at least ten years of schooling to reach this same probability.
Estimates of prevalence of chronic diseases in individuals aged 40 years or older showed signifi cant relative bias.Relative bias introduced by low coverage rates according to region were found to have different behavior, shown on Table 3.The Northern region and Central-west, Northeast and Southeast non-metropolitan regions, with home landline telephone coverage rates below 60%, showed relative bias above 1, thus reducing the pre-established 95% confi dence level to 83%.In the Northeast metropolitan region, this change was found to reach about 90%.In the Southeast, Southern and Central-west metropolitan regions, with coverage rates above 70%, associated bias was considered insignifi cant.

DISCUSSION
During the period studied, growth in home landline telephone coverage rates was found in Brazil, with higher indices for the Southern and Southeast metropolitan regions, though reaching mean coverage below 60% in 2003.In this way, Brazil is far distant from the coverage rates found in developed countries that use home landline telephones to conduct epidemiological surveys. 2,3nce 1987, problems associated with the exclusion of homes without telephone line access have been an object of study for publications and international scientifi c meetings aimed at the area of statistical methodology.According to Groves et al 2 (2001), errors caused by the exclusion of homes without a telephone line or with a telephone number unavailable in the telephone company's records are not insignifi cant.To compensate the exclusion of this population, adjusting the distribution of the sample with a telephone to the total population composition, according to characteristics associated to owning a telephone line and using weighting methods, has become a common practice.In this way, bias potentially associated with inferences drawn is expected to be reduced.In this study, after weighting the sample, it was observed that level of education, ethnicity and head of family owning a health care plan were associated with owning a telephone line in the urban Northern region.In the Northeast region, the same characteristics previously mentioned and the urban area variable were observed.In the Southeast, Southern and Central-west regions, all these characteristics were present, though differing in terms of the metropolitan region variable.
However, the Southern non-metropolitan region, with 60% of homes served by a telephone line, showed lower relative bias.Such result is caused by small differences between the prevalences of chronic diseases estimated for the strata with and without a home landline telephone.Moreover, this result emphasizes the importance that specifi c methodologies to treat estimates obtained in locations with lower indices should be studied and published in Brazilian scientifi c journals preferably.
The existence of a subgroup of the population living in homes not served by a telephone line, as well as their sociodemographic characteristics and geographic location, is a source of error associated with the exclusion of such homes, which may change the pre-established confi dence level for the estimates to be obtained.Thus, low home telephone line coverage rates can result in biased estimates not corresponding to the realities studied.
Results from this study indicate that the use of home telephone line records to conduct interviews in sampling surveys is only reliable for Brazilian regions with home landline telephone coverage rates above 70%.
Brazilian studies on alternative methodologies for the use of telephone surveys in areas with low home telephone line coverage should be the object of epidemiological research.

Figure 2 .
Figure 2. Probability of owning a home telephone line, according to ethnicity and number of years of schooling.Brazil, 1998-2003.

Table 2
Source: Instituto Brasileiro de Geografi a e Estatística (IBGE -Brazilian Institute of Geography and Statistics).Micro data from the Pesquisa Nacional por Amostra de Domicílios -1998 to 2003 (PNAD -National Household Sample Survey).City of Rio de Janeiro, 2003.Figure 1. Progress of coverage of homes with telephone lines per region.Brazil, 1998-2003.

Table 1 .
Sociodemographic characteristics of adults living in homes, according to ownership of a home telephone line.
Source: Instituto Brasileiro de Geografi a e Estatística (IBGE -Brazilian Institute of Geography and Statistics).Micro data from the Pesquisa Nacional por Amostra de Domicílios-1998 to 2003 (PNAD -National Household Sample Survey).City of Rio de Janeiro, 2003.

Table 3 .
Coverage rates and relative bias of prevalences estimated for chronic diseases in adults aged 40 years and older, according to region.Brazil, 1998-2003 Instituto Brasileiro de Geografi a e Estatística (IBGE -Brazilian Institute of Geography and Statistics).Micro data from the Pesquisa Nacional por Amostra de Domicílios-1998 to 2003 (PNAD -National Household Sample Survey).City of Rio de Janeiro, 2003.Source: Instituto Brasileiro de Geografi a e Estatística (IBGE -Brazilian Institute of Geography and Statistics).Micro data from the Pesquisa Nacional por Amostra de Domicílios -1998 to 2003 (PNAD -National Household Sample Survey).City of Rio de Janeiro, 2003