[1] FAC MED PARIS SUD, F-94807 VILLEJUIF, FRANCE
[2] UNIV KUOPIO, DEPT MED, SF-70210 KUOPIO, FINLAND
[3] UCL, SCH MED, DEPT EPIDEMIOL & PUBL HLTH, LONDON W1N 8AA, ENGLAND
[4] UNITED MED & DENT SCH GUYS & ST THOMAS HOSP, DEPT PUBL HLTH MED, LONDON SE1 7EH, ENGLAND