Further, the observational study design creates difficulties in establishing a causal relationship between infection status and birth outcomes because of possible confounding factors.
Analyses were also repeated with a term for drinking-water source (surface vs. ground) because of possible confounding by chlorination by-products present in one treated surface water supply.
Because of possible confounding from age related weight change in the elderly population, we used follow-up data only up to 1998 as the mean age of our study samples was over 65 after 1998.
However, the literature suggests that the relationship between length of service (tenure) and turnover is complex, because of possible confounding factors such as context, work environment, experience, and age (4, 13, 17, 25, 29, 42, 43, 45).
Recently the causality of associations between U-Cd and biomarkers of kidney effects in populations with low levels of exposure has been questioned because of possible confounding by smoking or physiological sources of variability (Bernard 2008; Chaumont et al. 2010, 2012; Haddam et al. 2011).
Because of possible confounding by boosting from recent reinfections, such studies are only possible in areas with very low P. falciparum transmission, or in people living in malaria‐free countries that were either exposed to malaria experimentally or naturally during travel in endemic areas 42.
