The primary outcome was mothers' breastfeeding status and expressing pattern at 6 weeks postpartum, and the latter was an exposure variable in this study.
24 Given that we explored bidirectional associations between common mental disorders and obesity, caseness (and obesity) could be considered an exposure variable or an outcome.
For quantitative analysis, the integrated TCDD concentration over time was used as an exposure variable, which was calculated using results from half-life estimation for TCDD and workplace history data.
In studies where fish consumption is used as an exposure variable, observed benefits are often attributed to the presence of the fatty acids, despite the fact that the concentration of fatty acids in fish varies considerably depending on the species, season, etc.
These factors can be used as an exposure variable in statistical models (Maresca et al. 2015).
Soil Pb may be an important exposure variable in these environments, possibly overwhelming exposure to other sources of Pb.
Also, no studies were located in which low formal education had been investigated as an exposure variable, preventing comparison with current finding of higher prevalence of heavy periods among less educated women.
Swimming once a week or more has also been used as an exposure variable in other population-based studies [ 14], although it should be remembered that in our study, swimming was specified as in indoor pools.
For injuries of all types and that related to exercise, the total number of firefighters for each year, on the log scale, was considered as an exposure variable.
Therefore, breast milk expressing pattern at 6 weeks postpartum was selected as an exposure variable for the study.
