In situations where follow-up includes a mix of prevalent and incident exposures, as in many occupational studies, inclusion of prevalently exposed persons may greatly increase the statistical precision of the "late" exposure duration windows and/or the "long" time-since-first-exposure windows.
The inclusion of occupational controls on the right-hand side of wage equations has to be handled with care since both wages and occupations may be a proxy for immigrants' labour market performance.
Still, as stressed in Chiswick and Miller (2007), the inclusion of occupational dummies may be justified when the focus of the analysis is on the channels through which earnings progression occurs (Groshen 1991).
Support for this is found in studies from the United States and Australia in which inclusion of occupational and household activities eliminated SES differentials in PA among men but not among women [ 12, 13].
In Germany the case control mean exposure estimates were substantially smaller than the survey estimates (median estimates were not available for the case control study), which could be due to regional differences and the inclusion of occupational exposures in the survey estimates.
A stepwise procedure of multiple logistic regressions with pre-specified steps was used to analyse alterations of the associations between respiratory health indicators and SES (model I) by consecutively additional inclusion of occupational exposure (model II), tobacco smoke (model III) and environmental air pollution exposure (model IV) into the regression models.
As we and others [ 3, 11, 12] have noted, some of the principal drivers behind such groups' professionalization projects include: the pursuit of increased occupational credibility, more extensive inclusion within third party and/or public health insurance programs, and the promotion of higher or more consistent training standards.
A limitation of this study is the small sample size and the inclusion of only occupational therapists in the public health sector in one area of South Africa.
Future studies investigating the role of filaggrin in occupational HE should consider the inclusion of copy number variations in their analysis.
Information regarding the treatment program followed by each participant is collected by the occupational health physicians at the inclusion visit and during the follow-up visits.
