National health surveys by mail or home interview: eVects on response

Study objective—To study the eVect of using a mail questionnaire or home interviews on the size and the selectivity of response to national health surveys. Design—The interview survey and the mail survey were both carried out in the same country (the Netherlands) using the same sample frame, the same study period (1998) and collected partly the same data on demographic, socioeconomic and health characteristics. Setting—The Netherlands. Participants—Dutch non-institutionalised inhabitants aged 25 years and over. Main results—Response to the mail survey was lower (46.9%, n=3664) than to the interview survey (58.5%, n=6061). The mail survey gave higher response rates for women and lower response rates for persons with lower levels of education. Respondents to the mail survey reported lower rates of smoking but a slightly worse health status and higher figures on the use of health care services. No diVerences by method of data collection were found for age, marital status, region, household composition, work status and categories of body mass index. Conclusion—Although the response of the mail survey was lower than the home interview survey, respondents showed generally small diVerences, with exception of level of education. (J Epidemiol Community Health 2001;55:408–413) National health surveys are the main sources for data on many (public) health indicators. Data collection can be carried out by face to face interviews, telephone interviews, mail questionnaires or a combination of methods. In the Netherlands we often use face to face interviews carried out at home or mail questionnaires. The choice of the mode of data collection is determined by several factors, including available resources and response expectations. Face to face interviews surveys are much more expensive than mail surveys. Several eVects of mode of data collection on response are known. Firstly, it is generally thought that response rates are better for interview surveys than mail surveys, some evidence for elderly people is available. Secondly, those who respond to interview surveys can be diVerent from those who respond to mail surveys. There is some evidence that lower socioeconomic classes are under-represented in mail surveys compared with interview surveys. Thirdly, people can respond diVerently to questions on paper than to questions asked by an interviewer. For instance, for some disability indicators it is known that systematically higher prevalences are found using self administered questionnaires compared with interviews. The same is found for other health indicators. In addition, questions that can be aVected by social desirability, for example, alcohol consumption, using of car belts, are suspected to do better in self administered questionnaires than in face to face interviews. However, information on height, household composition, work status is considered not to be aVected by mode of data collection. For this paper we were able to study the response to a health mail survey and to a health interview survey, both using the same sample frame (population register), the same target population (the Dutch non-institutionalised population) and mainly the same topics and questions. The question of our study is: does using a mailed survey or an interview survey lead to diVerent response groups? According to the (limited) data in the literature we expect (1) the response on the mail survey to be lower than on the interview survey, (2) that respondents to the mail survey with lower educational levels are underrepresented and (3) that population estimates on non-mode dependent questions such as work status, number of persons in the household, height and weight, are not aVected by mode of data collection. (4) Our last hypothesis is that population-based estimates of health indicators based on mail survey will represent a less favourable health status compared with the interview survey. Methods Two health surveys carried out in the Netherlands were analysed (1) the Netherlands Health Interview Survey (NetHIS) of 1998, which uses face to face interviews at home carried out by trained interviewers combined with a paper questionnaire, and (2) the baseline of the Dutch Musculoskeletal Complaints and Consequences Cohort study (DMC3-study), which uses mail questionnaires. General characteristics of the two studies are presented in table 1. The NetHIS is a continuous survey started in 1981 and carried out by Statistics Netherlands. From 1997 the NetHIS is one module of the integrated system of face to face interviews of Statistics Netherlands. The DMC3-study is carried out by the National Institute of Public Health and the Environment in collaboration with Statistics Netherlands. For the two surveys the same sample frame (population register) and sample method were J Epidemiol Community Health 2001;55:408–413 408 National Institute of Public Health and the Environment, the Netherlands