Affiliations: [a] Canadian Centre for Health and Safety in Agriculture, College of Medicine, University of Saskatchewan, Saskatchewan, Canada | [b] Department of Community Health and Epidemiology, College of Medicine, University of Saskatchewan, Saskatchewan, Canada | [c] Department of Public Health Sciences, School of Public Health, University of Alberta, Alberta, Canada | Division of Biostatistics, Food and Drug Administration, Rockville, MD, USA
Correspondence:
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Corresponding author: Dr. Punam Pahwa, Associate Professor, Canadian Centre for Health and Safety in Agriculture, Wing 3E, Royal University Hospital, 103 Hospital Drive, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OW8. Tel.: +1 306 966 8300; Fax: +1 306 966 8799; E-mail: [email protected]
Abstract: Correlated survival data analysis techniques were utilized to determine the efficacy of such techniques to analyze correlated airway responsiveness data. When the survival data are correlated, standard maximum likelihood estimates of the regression coefficients obtained by using the Cox’s model are consistent, but the estimates of standard errors may not be valid or consistent due to within-subject dependencies and hence give rise to wrong interpretations. We used jackknife, bootstrap and the method proposed by Wei, Lin and Weissfeld (WLW) to obtain robust estimates for the standard errors. The data analyzed in this report were obtained from a longitudinal study conducted to investigate the respiratory health effects of initial exposure to grain dust among workers commencing employment in the grain industry in the province of Saskatchewan, Canada. Bronchial responsiveness was determined by histamine inhalation test administered at four different time points. The provocation concentration (PC20) of inhaled histamine that produced 20% fall in the lung function parameter forced expired volume in one second (FEV1) was used as an indicator of bronchial responsiveness. Failure was defined as the dose of histamine at which a 20% fall in FEV1 occurred. In provocation tests, censored data may occur due to the failure to reach a determined end point. Censoring occurred when a subject did not have 20% fall in FEV1 with the maximum histamine dose of 8 mg. Significant predictors of bronchial responsiveness were FEV1, wheezing and height. The estimates of standard errors were similar for jackknife, bootstrap, and WLW, but different from those obtained using standard maximum likelihood method.
