Investigating the Effect of Nonparticipation Using a Population-Based Case–Control Study on Myocardial Infarction ELISABETE RAMOS, MPH, CARLA LOPES, PHD, AND HENRIQUE BARROS, MD, PHD PURPOSE: To characterize non-participants in a population-based study on cardiovascular diseases and investigate the effect of non-participation on risk estimates for myocardial infarction. METHODS: Using random digit dialing we obtained full information for 1054 adults (60.8% female), while 345 eligible individuals (72.5% female) declined the invitation to participate, but answered a limited set of questions by telephone. Risk of myocardial infarction was estimated using 474 cases (19.4% females) admitted with a first acute myocardial infarction. RESULTS: Participation proportion was 99.0% for cases and 70.0% for population controls. Population non-participants were older (61.6 vs. 58.5 years, for males, and 62.9 vs. 57.7 years for females) and more frequently women (66.3% vs. 74.7%, p  0.001); males tended to be non-drinkers and to have had a blood test during the previous year; females were additionally more often non-smokers. Crude and adjusted risk estimates for myocardial infarction were generally similar regardless of considering the information provided by non-participants. CONCLUSIONS: In this South European population, demographic and social characteristics associated with the decision to participate in a community investigation were different from those usually described in Northern European or American populations. However, their characteristics did not influence the direction or the magnitude of myocardial infarction risk estimates. Ann Epidemiol 2004;14:437–441.  2004 Elsevier Inc. All rights reserved. KEY WORDS: Response Bias, Myocardial Infarction, Participation. INTRODUCTION Recruitment strategies to avoid participation bias are essen- tial (1, 2). The ideal situation would be to keep refusal as close to zero as possible but in general not every eligible person will participate. Participation proportion is related to design options, study topics, and cultural background of the population (3). As the latter might add specific con- cerns we cannot assume that determinants of non-participa- tion operate everywhere in the same way. When refusal to participate is differential there is a partic- ular risk of biased estimates. This is frequently expected when population-based control groups are used or general sampling procedures, such as random digit dialing, is the option (4–6). If the decision to participate in an epidemiological inves- tigation is associated both with the exposure and the disease under study, biased risk estimates can result. However, the impact of this potential bias is rarely quantified because differences between participants and non-participants are From the Department of Hygiene and Epidemiology, University of Porto Medical School, Porto, Portugal. Address correspondence to: Dr. Henrique Barros, Servic ¸o de Higiene e Epidemiologia, Faculdade de Medicina do Porto, 4200-319 Porto, Portugal. Tel.: +351-22-550-7597; Fax: +351-22-509-5618. E-mail: hbarros@med.up.pt This study was supported by a grant from FCT (PRAXIS/2/2.1/SAU/ 1332/1995). Received October 9, 2002; accepted September 16, 2003.  2004 Elsevier Inc. All rights reserved. 1047-2797/04/$–see front matter 360 Park Avenue South, New York, NY 10010 doi:10.1016/j.annepidem.2003.09.013 usually unknown due to the very nature of the non- response problem. Frequently the available information from non-participants is limited to a narrow range of social and demographic variables or data on vital statistics obtain- able regardless of individual agreement (3, 7, 8). However, non-participants probably often differ in other important respects, and it is crucial to identify such differences to interpret study results correctly (3, 5, 9). In the present study we compared demographic, behav- ioral, and clinical characteristics of full participants and those who only agreed to provide telephone information on a limited set of variables. Considering these specific variables we determined the expected effect of non-partici- pation on risk estimates for acute myocardial infarction. PARTICIPANTS AND METHODS This study was part of a previously described population- based case–control investigation on risk factors for myocar- dial infarction, the EPIcardis study (10, 11), assessing persons older than 39 years. Cases and controls were perma- nent residents in the catchments area of Hospital de S. Joa ˜o, Porto. All myocardial infarction patients living in the defined area are admitted to the cardiology wards of the hospital or referred to the cardiology department if for any reason the initial acute care is provided elsewhere.