Original articles
Healthy Volunteer Effect in Industrial Workers

https://doi.org/10.1016/S0895-4356(99)00070-0Get rights and content

Abstract

Volunteers for epidemiological research, have lower mortality rates than non-volunteers, thereby producing a bias referred to as the “healthy volunteer effect” (HVE). Occupationally active persons have been similarly shown to have a reduced mortality relatively to the general population (the “healthy worker effect”). To determine whether a HVE exists in occupationally active persons, we followed for 8 years a cohort of Israeli male industrial employees, of whom 71.6% agreed to participate in 1985 in screening examinations for cardiovascular disease. We calculated standardized mortality ratios (SMRs) of the entire cohort relative to the general population, and compared the mortality among participants with that of the non-participants. Over 8 years follow up, SMRs were 78% for the entire cohort, 71% for participants and 99% for non-participants. Participants were older than non-participants and worked more commonly in smaller factories. A proportional hazard model indicated that after adjusting for these variables, the all cause mortality hazard ratio among participants and non-participants was 0.69 (95% CI = 0.51–0.94). During the first and last two years of the 8-year follow-up there were 39.6 and 30.0 age-adjusted deaths per 10,000 person-years among participants, and 58.6 and 51.5 respectively among non-participants. We conclude that the HVE occurs in occupationally active persons, and that it may persist for up to 8 years follow-up.

Introduction

Increased mortality rates have been repeatedly observed among non-responders to invitations to participate in epidemiological surveys. Mortality among Seventh-Day Adventist's [1] or among Japanese men in Honolulu [2] who did not respond to a mailed questionnaire, was higher than that among those who did. This “healthy volunteer effect” (HVE) has been confirmed in several other studies 3, 4, 5, 6, 7, 8 and emerges as an important source of bias in epidemiological research.

The workplace is a convenient setting for epidemiological research. Testing is usually done on site and does not require from participants to invest leisure or work time. However, occupationally active persons are a selected population capable of holding a job. They have been found to be healthier than the general population, thereby producing the “healthy worker effect” [9]. Whether the HVE applies also to such cohorts is uncertain. The only attempt we know of, to detect a HVE in occupationally active persons was carried out in 1957, and found no differences in mortality between participants and non-participants in an epidemiological survey [10].

The objective of the following study was to determine whether the HVE exists in a cohort of industrial workers. We compared the mortality and causes of death during 8 years of 3,795 male workers who responded in 1985 to an invitation to participate in a prospective study, with those of 1,507 workers who did not.

Section snippets

Study Population

An effort was made in 1985 to identify all male industrial employees of 21 plants throughout Israel. A total of 5547 employees of textile, heavy industry (tire and metal), light industries (printing and home products manufacture), and others (furniture, electronics, and food), were located and offered free of charge screening examinations for selected risk factors for cardiovascular disease (CVD). Nearly 70% were blue-collar workers. Of the 5547 workers, 3816 (68.8%) responded and were examined

Healthy Worker Effect

The 8-year cumulative mortality from cancer, CVD and other causes in the entire cohort of employees who were offered the examinations increased with age (Table 2). The age-adjusted standardized all cause mortality ratio in the entire cohort was 78 (95% CI: 68–89), that for CVD mortality was 75 (95% CI: 60–92), and that for cancer mortality was 94 (95% CI: 73–119).

Healthy Volunteer Effect

The SMRs were 99% (95% CI: 78–124) for non-participants and 71% (95% CI: 60–83) for participants (Table 3). The differences in

Discussion

The main finding of this study is the reduced mortality risk among subjects who volunteered to participate in screening examinations as compared to those who did not, and to the general population. While this “healthy volunteer effect” has previously been observed in other populations 1, 2, 3, 4, 5, 6, 7, 8, this study is the first to demonstrate such an effect in a cohort of industrial workers, a select population that was shown to be healthier than the general population to begin with. As

Acknowledgements

The authors thank the Committee for Preventive Action and Research in Occupational Health, Ministry of Labor and Social Welfare, Jerusalem, Israel for their support, and also Professor Manfred Green who was instrumental in instituting this study.

References (12)

There are more references available in the full text version of this article.

Cited by (94)

  • Wearables, smartphones, and artificial intelligence for digital phenotyping and health

    2020, Digital Health: Mobile and Wearable Devices for Participatory Health Applications
  • In utero metal exposures measured in deciduous teeth and birth outcomes in a racially-diverse urban cohort

    2019, Environmental Research
    Citation Excerpt :

    We have previously shown that children from WHEALS who donated a tooth to the study were of slightly older gestational age at delivery (39.1 ± 1.3 weeks compared to 38.7 ± 1.8 weeks) and higher birth weight Z-score (0.06 ± 0.95 compared to −0.15 ± 0.99 weeks) than WHEALS children who did not donate a tooth to the study (Cassidy-Bushrow et al., 2017). As reported elsewhere, the propensity for participation in research studies by healthier volunteers (with respect to both outcomes of interest and potential exposures) may bias the results of a study (Froom et al., 1999), potentially toward the null. There are known sex differences in the vulnerability to prenatal exposures (DiPietro and Voegtline, 2017); we found evidence for sex-specific effects of tooth manganese on birth weight Z-score in stratified models but not when tested with interaction terms, which may suggest that sex acts as an effect modifier rather than an interaction variable (VanderWeele, 2009).

  • Usefulness of Non–High-Density Lipoprotein Cholesterol as a Predictor of Cardiovascular Disease Mortality in Men in 22-Year Follow-Up

    2017, American Journal of Cardiology
    Citation Excerpt :

    However, data collected during the first and second phases of the CORDIS study were crosschecked to ensure reliability. A nondifferential information bias in outcome analysis may stem from mortality data obtained from NDRs; however, verification of NDR coding has previously indicated a coding accuracy of 91%.28 The authors have no conflicts of interest to disclose.

View all citing articles on Scopus
View full text