Mortality among participants and non-participants in a prospective cohort study
Socioeconomic position and lifestyle often affect participation in scientific studies. The authors investigated differences in overall and cause-specific mortality between participants and non-participants in the prospective Danish cohort study “Diet, Cancer and Health” and the association between non-participation and mortality by socioeconomic position. A total of 80,996 men and 79,729 women aged 50–64 years, were invited. The authors obtained register data on education, income, death and cause-specific mortality for participants and non-participants and used survival curves to examine differences in overall mortality. Poisson regression models were used to estimate the mortality rate ratio (MRR) by socioeconomic group and by cause of death of participants and non-participants. After a median follow-up of 13 years (5–95 percentiles, 5–14 years), the MRRs for overall mortality among non-participants were 2.09 (95 % CI 1.99–2.14) and 2.29 (95 % CI 2.19–2.40) among men and women, respectively compared with participants. After adjusting for socioeconomic position, the MRRs changed to 1.73 (95 % CI 1.66–1.79) and 2.10 (95 % CO 2.01–2.20) among men and women, respectively. The MRRs did not level out after up to 15 years of follow-up. The MRRs were all significantly increased and ranged from 1.51 to 4.28 for men, depending on the cause of death, and from 1.60 to 3.99 for women. Clear differences in mortality from all investigated causes of death were found between participants and non-participants, which persisted after up to 15 years of follow-up. Socioeconomic position had little effect on this result.
KeywordsParticipation Cohort study Overall mortality Cause-specific mortality Socioeconomic position Lifestyle
We thank Katja Boll for technical support. The “Diet, Cancer and Health” study was established by Anne Tjønneland and Kim Overvad in 1993–1995 and funded by the Danish Cancer Society. This work was supported by the Health Insurance Foundation grant no. 2006B95 and the Danish Cancer Society Scientific Committee grant no. DP06035.
Conflict of interest
- 1.Rothman KJ, Greenland S, Lash T. Modern epidemiology. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 2008.Google Scholar
- 3.Jousilahti P, Salomaa V, Kuulasmaa K, Niemela M, Vartiainen E. Total and cause specific mortality among participants and non-participants of population based health surveys: a comprehensive follow up of 54,372 Finnish men and women. J Epidemiol Community Health. 2005;59(4):310–5.PubMedCrossRefGoogle Scholar
- 6.Andersen LB, Vestbo J, Juel K, Bjerg AM, Keiding N, Jensen G, et al. A comparison of mortality rates in three prospective studies from Copenhagen with mortality rates in the central part of the city, and the entire country. Copenhagen Center for Prospective Population Studies. Eur J Epidemiol. 1998;14(6):579–85.PubMedCrossRefGoogle Scholar
- 7.Bengtsson C, Ahlqwist M, Andersson K, Bjorkelund C, Lissner L, Soderstrom M. The Prospective Population Study of Women in Gothenburg, Sweden, 1968–1969 to 1992–1993. A 24-year follow-up study with special reference to participation, representativeness, and mortality. Scand J Prim Health Care. 1997;15(4):214–9.PubMedCrossRefGoogle Scholar
- 12.Hara M, Sasaki S, Sobue T, Yamamoto S, Tsugane S. Comparison of cause-specific mortality between respondents and nonrespondents in a population-based prospective study: 10-year follow-up of JPHC Study Cohort I. Japan Public Health Center. J Clin Epidemiol. 2002;55(2):150–6.PubMedCrossRefGoogle Scholar
- 16.Tjonneland A, Olsen A, Boll K, Stripp C, Christensen J, Engholm G, et al. Study design, exposure variables, and socioeconomic determinants of participation in Diet, Cancer and Health: a population-based prospective cohort study of 57,053 men and women in Denmark. Scand J Public Health. 2007;35(4):432–41.PubMedCrossRefGoogle Scholar