RT Journal Article SR Electronic T1 Approaches for estimating prevalence ratios JF Occupational and Environmental Medicine JO Occup Environ Med FD BMJ Publishing Group Ltd SP 501 OP 506 DO 10.1136/oem.2007.034777 VO 65 IS 7 A1 J A Deddens A1 M R Petersen YR 2008 UL http://oem.bmj.com/content/65/7/501.abstract AB