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A comparison between standard methods and structural nested modelling when bias from a healthy worker survivor effect is suspected: an iron-ore mining cohort study
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Oncology.
Umeå University, Faculty of Medicine, Department of Public Health and Clinical Medicine, Occupational and Environmental Medicine. Sundsvall Hosp, Dept Occupat & Environm Med, Sundsvall, Sweden.
2015 (English)In: Occupational and Environmental Medicine, ISSN 1351-0711, E-ISSN 1470-7926, Vol. 72, no 7, 536-542 p.Article in journal (Refereed) Published
Abstract [en]

Objectives Iron-ore miners are exposed to extremely dusty and physically arduous work environments. The demanding activities of mining select healthier workers with longer work histories (ie, the Healthy Worker Survivor Effect (HWSE)), and could have a reversing effect on the exposure-response association. The objective of this study was to evaluate an iron-ore mining cohort to determine whether the effect of respirable dust was confounded by the presence of an HWSE. Methods When an HWSE exists, standard modelling methods, such as Cox regression analysis, produce biased results. We compared results from g-estimation of accelerated failure-time modelling adjusted for HWSE with corresponding unadjusted Cox regression modelling results. Results For all-cause mortality when adjusting for the HWSE, cumulative exposure from respirable dust was associated with a 6% decrease of life expectancy if exposed >= 15 years, compared with never being exposed. Respirable dust continued to be associated with mortality after censoring outcomes known to be associated with dust when adjusting for the HWSE. In contrast, results based on Cox regression analysis did not support that an association was present. Conclusions The adjustment for the HWSE made a difference when estimating the risk of mortality from respirable dust. The results of this study, therefore, support the recommendation that standard methods of analysis should be complemented with structural modelling analysis techniques, such as g-estimation of accelerated failure-time modelling, to adjust for the HWSE.

Place, publisher, year, edition, pages
2015. Vol. 72, no 7, 536-542 p.
Keyword [en]
Cox regression, occupation, standardized incidence ratio, standardized mortality ratio
National Category
Environmental Health and Occupational Health
Research subject
Occupational and Environmental Medicine
Identifiers
URN: urn:nbn:se:umu:diva-81789DOI: 10.1136/oemed-2014-102251ISI: 000356298200012OAI: oai:DiVA.org:umu-81789DiVA: diva2:658571
Note

Originally published in manuscript form with the title A comparison between standard methods and structural nested modeling when adjusting for the healthy worker survivor effect. 

Available from: 2013-10-22 Created: 2013-10-22 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Strategies for assessing health risks from two occupational cohorts within the domain of northern Sweden
Open this publication in new window or tab >>Strategies for assessing health risks from two occupational cohorts within the domain of northern Sweden
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Strategier vid utvärdering av hälsorisker baserade på två arbetarekohorter från norra Sverige
Abstract [en]

Background Studies based on a cohort design requires access to both subject-specific and period-specific information. In order to conduct an occupational cohort study, access to exposure information and the possibility and permission to link information on outcomes from other registers are generally necessary. The analysis phase is also aggravated by its added complexity because of the longitudinal dimension of the cohort’s data.This thesis aims at increasing the knowledge on hazards from work on fatalities and cancer within the domain of cohort studies on miners and metal refiners and to study the complexity of the analysis by discussing and suggesting analytical strategies.

Methods The study population for this thesis consisted of a cohort of 2264 blue-collar aluminium smelter workers (paper I) and a cohort of 13000 blue-collar iron-ore miners (papers II-IV), both followed for over 50 years. The outcomes were collected from the Swedish Cause of Death Register and the Swedish Cancer Register. The primary methods of analysis were either Standardized Morbidity Ratios (SMR) or internal comparisons based on Cox or Poisson regression modeling. In paper IV, a g-estimation based on an accelerated failure-time model was performed to estimate the survival ratio.

Results The results from paper I suggested that working as a blue-collar worker metal refiner was associated with increased rates of incidental lung cancer. Elevated rates among short term workers were observed for several outcomes. Paper I also showed that the choice of reference population when calculating SMR could influence the conclusions of the results. In paper II, several outcomes were elevated among the miners compared to the reference population from northern Sweden. However, no outcome except lung cancer was associated with cumulative employment time. The most recurrent pattern of the results was the negative association between cumulative employment time underground and several outcomes. The results from paper III showed that cumulative employment time working outdoors was associated with increased rates of cerebrovascular disease mortality. However, employment with heavy physical workloads did not explain the previously observed decreasing rates in the selected groups of outcomes. The adjustment for the healthy worker survivor effect by g-estimation in paper IV suggested that exposure from respirable dust was associated with elevated mortality risks that could not be observed with standard analytical methods.

Conclusion Our studies found several rates from the cohorts that were elevated compared to external refererence populations but also that long term employments generally were associated with decreasing rates. Furthermore, incidental lung cancer rates was found elevated for the metal refiners. Among the miners, mortality rates of cerebrovascular diseases depended on if work was performed outdoor (higher rates) or underground (lower rates). Methodologically, this thesis has discussed different analytical strategies for handling confounding in occupational cohort studies. Paper IV showed that the healthy worker survivor effect could be adjusted for by performing g-estimation.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet, 2013. 69 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1605
Keyword
Cohort, mortality, incidence, risk, rate, cancer, cerebrovascular diseases, exposure, occupational, mining, industry, worker, Poisson regression, Cox regression, SMR, causal inference, G-estimation
National Category
Environmental Health and Occupational Health
Research subject
Occupational and Environmental Medicine; Epidemiology
Identifiers
urn:nbn:se:umu:diva-81764 (URN)978-91-7459-742-4 (ISBN)
Public defence
2013-11-15, Tripple helix, Samverkanshuset, Umeå Universitet, Umeå, 13:00 (Swedish)
Opponent
Supervisors
Available from: 2013-10-25 Created: 2013-10-21 Last updated: 2013-10-25Bibliographically approved

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Björ, OveDamber, LenaJonsson, HåkanNilsson, Tohr

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