RT Journal Article SR Electronic T1 Confounding and confounders JF Occupational and Environmental Medicine JO Occup Environ Med FD BMJ Publishing Group Ltd SP 227 OP 234 DO 10.1136/oem.60.3.227 VO 60 IS 3 A1 R McNamee YR 2003 UL http://oem.bmj.com/content/60/3/227.abstract AB