RT Journal Article SR Electronic T1 Overview of study design in clinical epidemiology JF Sexually Transmitted Infections JO Sex Transm Infect FD BMJ Publishing Group Ltd SP 244 OP 247 DO 10.1136/sti.76.4.244 VO 76 IS 4 A1 J M Stephenson A1 A Babiker YR 2000 UL http://sti.bmj.com/content/76/4/244.abstract AB